diff --git a/src/Polly/Caching/AsyncCacheSyntax.cs b/src/Polly/Caching/AsyncCacheSyntax.cs
index d3c6ce6032b..2e5d4f24fa0 100644
--- a/src/Polly/Caching/AsyncCacheSyntax.cs
+++ b/src/Polly/Caching/AsyncCacheSyntax.cs
@@ -1,4 +1,5 @@
-namespace Polly;
+#nullable enable
+namespace Polly;
 
 public partial class Policy
 {
@@ -13,7 +14,7 @@ public partial class Policy
     /// <param name="onCacheError">Delegate to call if an exception is thrown when attempting to get a value from or put a value into the cache, passing the execution context, the cache key, and the exception.</param>
     /// <returns>The policy instance.</returns>
     /// <exception cref="ArgumentNullException">cacheProvider</exception>
-    public static AsyncCachePolicy CacheAsync(IAsyncCacheProvider cacheProvider, TimeSpan ttl, Action<Context, string, Exception> onCacheError = null)
+    public static AsyncCachePolicy CacheAsync(IAsyncCacheProvider cacheProvider, TimeSpan ttl, Action<Context, string, Exception>? onCacheError = null)
         => CacheAsync(cacheProvider, new RelativeTtl(ttl), DefaultCacheKeyStrategy.Instance.GetCacheKey, onCacheError);
 
     /// <summary>
@@ -28,7 +29,7 @@ public static AsyncCachePolicy CacheAsync(IAsyncCacheProvider cacheProvider, Tim
     /// <returns>The policy instance.</returns>
     /// <exception cref="ArgumentNullException">cacheProvider</exception>
     /// <exception cref="ArgumentNullException">ttlStrategy</exception>
-    public static AsyncCachePolicy CacheAsync(IAsyncCacheProvider cacheProvider, ITtlStrategy ttlStrategy, Action<Context, string, Exception> onCacheError = null)
+    public static AsyncCachePolicy CacheAsync(IAsyncCacheProvider cacheProvider, ITtlStrategy ttlStrategy, Action<Context, string, Exception>? onCacheError = null)
         => CacheAsync(cacheProvider, ttlStrategy, DefaultCacheKeyStrategy.Instance.GetCacheKey, onCacheError);
 
     /// <summary>
@@ -44,7 +45,7 @@ public static AsyncCachePolicy CacheAsync(IAsyncCacheProvider cacheProvider, ITt
     /// <returns>The policy instance.</returns>
     /// <exception cref="ArgumentNullException">cacheProvider</exception>
     /// <exception cref="ArgumentNullException">cacheKeyStrategy</exception>
-    public static AsyncCachePolicy CacheAsync(IAsyncCacheProvider cacheProvider, TimeSpan ttl, ICacheKeyStrategy cacheKeyStrategy, Action<Context, string, Exception> onCacheError = null)
+    public static AsyncCachePolicy CacheAsync(IAsyncCacheProvider cacheProvider, TimeSpan ttl, ICacheKeyStrategy cacheKeyStrategy, Action<Context, string, Exception>? onCacheError = null)
         => CacheAsync(cacheProvider, new RelativeTtl(ttl), cacheKeyStrategy.GetCacheKey, onCacheError);
 
     /// <summary>
@@ -61,7 +62,7 @@ public static AsyncCachePolicy CacheAsync(IAsyncCacheProvider cacheProvider, Tim
     /// <exception cref="ArgumentNullException">cacheProvider</exception>
     /// <exception cref="ArgumentNullException">ttlStrategy</exception>
     /// <exception cref="ArgumentNullException">cacheKeyStrategy</exception>
-    public static AsyncCachePolicy CacheAsync(IAsyncCacheProvider cacheProvider, ITtlStrategy ttlStrategy, ICacheKeyStrategy cacheKeyStrategy, Action<Context, string, Exception> onCacheError = null)
+    public static AsyncCachePolicy CacheAsync(IAsyncCacheProvider cacheProvider, ITtlStrategy ttlStrategy, ICacheKeyStrategy cacheKeyStrategy, Action<Context, string, Exception>? onCacheError = null)
     {
         if (cacheProvider == null) throw new ArgumentNullException(nameof(cacheProvider));
         if (ttlStrategy == null) throw new ArgumentNullException(nameof(ttlStrategy));
@@ -86,7 +87,7 @@ public static AsyncCachePolicy CacheAsync(IAsyncCacheProvider cacheProvider, ITt
     /// <returns>The policy instance.</returns>
     /// <exception cref="ArgumentNullException">cacheProvider</exception>
     /// <exception cref="ArgumentNullException">cacheKeyStrategy</exception>
-    public static AsyncCachePolicy CacheAsync(IAsyncCacheProvider cacheProvider, TimeSpan ttl, Func<Context, string> cacheKeyStrategy, Action<Context, string, Exception> onCacheError = null)
+    public static AsyncCachePolicy CacheAsync(IAsyncCacheProvider cacheProvider, TimeSpan ttl, Func<Context, string> cacheKeyStrategy, Action<Context, string, Exception>? onCacheError = null)
         => CacheAsync(cacheProvider, new RelativeTtl(ttl), cacheKeyStrategy, onCacheError);
 
     /// <summary>
@@ -103,7 +104,7 @@ public static AsyncCachePolicy CacheAsync(IAsyncCacheProvider cacheProvider, Tim
     /// <exception cref="ArgumentNullException">cacheProvider</exception>
     /// <exception cref="ArgumentNullException">ttlStrategy</exception>
     /// <exception cref="ArgumentNullException">cacheKeyStrategy</exception>
-    public static AsyncCachePolicy CacheAsync(IAsyncCacheProvider cacheProvider, ITtlStrategy ttlStrategy, Func<Context, string> cacheKeyStrategy, Action<Context, string, Exception> onCacheError = null)
+    public static AsyncCachePolicy CacheAsync(IAsyncCacheProvider cacheProvider, ITtlStrategy ttlStrategy, Func<Context, string> cacheKeyStrategy, Action<Context, string, Exception>? onCacheError = null)
     {
         if (cacheProvider == null) throw new ArgumentNullException(nameof(cacheProvider));
         if (ttlStrategy == null) throw new ArgumentNullException(nameof(ttlStrategy));
diff --git a/src/Polly/Caching/CacheSyntax.cs b/src/Polly/Caching/CacheSyntax.cs
index 8ab89c07118..abc7a537b15 100644
--- a/src/Polly/Caching/CacheSyntax.cs
+++ b/src/Polly/Caching/CacheSyntax.cs
@@ -1,4 +1,5 @@
-namespace Polly;
+#nullable enable
+namespace Polly;
 
 public partial class Policy
 {
@@ -13,7 +14,7 @@ public partial class Policy
     /// <param name="onCacheError">Delegate to call if an exception is thrown when attempting to get a value from or put a value into the cache, passing the execution context, the cache key, and the exception.</param>
     /// <returns>The policy instance.</returns>
     /// <exception cref="ArgumentNullException">cacheProvider</exception>
-    public static CachePolicy Cache(ISyncCacheProvider cacheProvider, TimeSpan ttl, Action<Context, string, Exception> onCacheError = null)
+    public static CachePolicy Cache(ISyncCacheProvider cacheProvider, TimeSpan ttl, Action<Context, string, Exception>? onCacheError = null)
         => Cache(cacheProvider, new RelativeTtl(ttl), DefaultCacheKeyStrategy.Instance.GetCacheKey, onCacheError);
 
     /// <summary>
@@ -28,7 +29,7 @@ public static CachePolicy Cache(ISyncCacheProvider cacheProvider, TimeSpan ttl,
     /// <returns>The policy instance.</returns>
     /// <exception cref="ArgumentNullException">cacheProvider</exception>
     /// <exception cref="ArgumentNullException">ttlStrategy</exception>
-    public static CachePolicy Cache(ISyncCacheProvider cacheProvider, ITtlStrategy ttlStrategy, Action<Context, string, Exception> onCacheError = null)
+    public static CachePolicy Cache(ISyncCacheProvider cacheProvider, ITtlStrategy ttlStrategy, Action<Context, string, Exception>? onCacheError = null)
         => Cache(cacheProvider, ttlStrategy, DefaultCacheKeyStrategy.Instance.GetCacheKey, onCacheError);
 
     /// <summary>
@@ -44,7 +45,7 @@ public static CachePolicy Cache(ISyncCacheProvider cacheProvider, ITtlStrategy t
     /// <returns>The policy instance.</returns>
     /// <exception cref="ArgumentNullException">cacheProvider</exception>
     /// <exception cref="ArgumentNullException">cacheKeyStrategy</exception>
-    public static CachePolicy Cache(ISyncCacheProvider cacheProvider, TimeSpan ttl, ICacheKeyStrategy cacheKeyStrategy, Action<Context, string, Exception> onCacheError = null)
+    public static CachePolicy Cache(ISyncCacheProvider cacheProvider, TimeSpan ttl, ICacheKeyStrategy cacheKeyStrategy, Action<Context, string, Exception>? onCacheError = null)
         => Cache(cacheProvider, new RelativeTtl(ttl), cacheKeyStrategy.GetCacheKey, onCacheError);
 
     /// <summary>
@@ -61,7 +62,7 @@ public static CachePolicy Cache(ISyncCacheProvider cacheProvider, TimeSpan ttl,
     /// <exception cref="ArgumentNullException">cacheProvider</exception>
     /// <exception cref="ArgumentNullException">ttlStrategy</exception>
     /// <exception cref="ArgumentNullException">cacheKeyStrategy</exception>
-    public static CachePolicy Cache(ISyncCacheProvider cacheProvider, ITtlStrategy ttlStrategy, ICacheKeyStrategy cacheKeyStrategy, Action<Context, string, Exception> onCacheError = null)
+    public static CachePolicy Cache(ISyncCacheProvider cacheProvider, ITtlStrategy ttlStrategy, ICacheKeyStrategy cacheKeyStrategy, Action<Context, string, Exception>? onCacheError = null)
     {
         if (cacheProvider == null) throw new ArgumentNullException(nameof(cacheProvider));
         if (ttlStrategy == null) throw new ArgumentNullException(nameof(ttlStrategy));
@@ -86,7 +87,7 @@ public static CachePolicy Cache(ISyncCacheProvider cacheProvider, ITtlStrategy t
     /// <returns>The policy instance.</returns>
     /// <exception cref="ArgumentNullException">cacheProvider</exception>
     /// <exception cref="ArgumentNullException">cacheKeyStrategy</exception>
-    public static CachePolicy Cache(ISyncCacheProvider cacheProvider, TimeSpan ttl, Func<Context, string> cacheKeyStrategy, Action<Context, string, Exception> onCacheError = null)
+    public static CachePolicy Cache(ISyncCacheProvider cacheProvider, TimeSpan ttl, Func<Context, string> cacheKeyStrategy, Action<Context, string, Exception>? onCacheError = null)
         => Cache(cacheProvider, new RelativeTtl(ttl), cacheKeyStrategy, onCacheError);
 
     /// <summary>
@@ -103,7 +104,7 @@ public static CachePolicy Cache(ISyncCacheProvider cacheProvider, TimeSpan ttl,
     /// <exception cref="ArgumentNullException">cacheProvider</exception>
     /// <exception cref="ArgumentNullException">ttlStrategy</exception>
     /// <exception cref="ArgumentNullException">cacheKeyStrategy</exception>
-    public static CachePolicy Cache(ISyncCacheProvider cacheProvider, ITtlStrategy ttlStrategy, Func<Context, string> cacheKeyStrategy, Action<Context, string, Exception> onCacheError = null)
+    public static CachePolicy Cache(ISyncCacheProvider cacheProvider, ITtlStrategy ttlStrategy, Func<Context, string> cacheKeyStrategy, Action<Context, string, Exception>? onCacheError = null)
     {
         if (cacheProvider == null) throw new ArgumentNullException(nameof(cacheProvider));
         if (ttlStrategy == null) throw new ArgumentNullException(nameof(ttlStrategy));
diff --git a/src/Polly/Caching/CacheTResultSyntax.cs b/src/Polly/Caching/CacheTResultSyntax.cs
index dbddd3b39d7..4b192a8529d 100644
--- a/src/Polly/Caching/CacheTResultSyntax.cs
+++ b/src/Polly/Caching/CacheTResultSyntax.cs
@@ -1,4 +1,5 @@
-namespace Polly;
+#nullable enable
+namespace Polly;
 
 public partial class Policy
 {
@@ -13,7 +14,7 @@ public partial class Policy
     /// <param name="onCacheError">Delegate to call if an exception is thrown when attempting to get a value from or put a value into the cache, passing the execution context, the cache key, and the exception.</param>
     /// <returns>The policy instance.</returns>
     /// <exception cref="ArgumentNullException">cacheProvider</exception>
-    public static CachePolicy<TResult> Cache<TResult>(ISyncCacheProvider cacheProvider, TimeSpan ttl, Action<Context, string, Exception> onCacheError = null)
+    public static CachePolicy<TResult> Cache<TResult>(ISyncCacheProvider cacheProvider, TimeSpan ttl, Action<Context, string, Exception>? onCacheError = null)
     {
         if (cacheProvider == null) throw new ArgumentNullException(nameof(cacheProvider));
 
@@ -32,7 +33,7 @@ public static CachePolicy<TResult> Cache<TResult>(ISyncCacheProvider cacheProvid
     /// <returns>The policy instance.</returns>
     /// <exception cref="ArgumentNullException">cacheProvider</exception>
     /// <exception cref="ArgumentNullException">ttlStrategy</exception>
-    public static CachePolicy<TResult> Cache<TResult>(ISyncCacheProvider cacheProvider, ITtlStrategy ttlStrategy, Action<Context, string, Exception> onCacheError = null)
+    public static CachePolicy<TResult> Cache<TResult>(ISyncCacheProvider cacheProvider, ITtlStrategy ttlStrategy, Action<Context, string, Exception>? onCacheError = null)
     {
         if (cacheProvider == null) throw new ArgumentNullException(nameof(cacheProvider));
 
@@ -52,7 +53,7 @@ public static CachePolicy<TResult> Cache<TResult>(ISyncCacheProvider cacheProvid
     /// <returns>The policy instance.</returns>
     /// <exception cref="ArgumentNullException">cacheProvider</exception>
     /// <exception cref="ArgumentNullException">cacheKeyStrategy</exception>
-    public static CachePolicy<TResult> Cache<TResult>(ISyncCacheProvider cacheProvider, TimeSpan ttl, ICacheKeyStrategy cacheKeyStrategy, Action<Context, string, Exception> onCacheError = null)
+    public static CachePolicy<TResult> Cache<TResult>(ISyncCacheProvider cacheProvider, TimeSpan ttl, ICacheKeyStrategy cacheKeyStrategy, Action<Context, string, Exception>? onCacheError = null)
     {
         if (cacheProvider == null) throw new ArgumentNullException(nameof(cacheProvider));
 
@@ -73,7 +74,7 @@ public static CachePolicy<TResult> Cache<TResult>(ISyncCacheProvider cacheProvid
     /// <exception cref="ArgumentNullException">cacheProvider</exception>
     /// <exception cref="ArgumentNullException">ttlStrategy</exception>
     /// <exception cref="ArgumentNullException">cacheKeyStrategy</exception>
-    public static CachePolicy<TResult> Cache<TResult>(ISyncCacheProvider cacheProvider, ITtlStrategy ttlStrategy, ICacheKeyStrategy cacheKeyStrategy, Action<Context, string, Exception> onCacheError = null)
+    public static CachePolicy<TResult> Cache<TResult>(ISyncCacheProvider cacheProvider, ITtlStrategy ttlStrategy, ICacheKeyStrategy cacheKeyStrategy, Action<Context, string, Exception>? onCacheError = null)
     {
         if (cacheProvider == null) throw new ArgumentNullException(nameof(cacheProvider));
 
@@ -93,7 +94,7 @@ public static CachePolicy<TResult> Cache<TResult>(ISyncCacheProvider cacheProvid
     /// <returns>The policy instance.</returns>
     /// <exception cref="ArgumentNullException">cacheProvider</exception>
     /// <exception cref="ArgumentNullException">cacheKeyStrategy</exception>
-    public static CachePolicy<TResult> Cache<TResult>(ISyncCacheProvider cacheProvider, TimeSpan ttl, Func<Context, string> cacheKeyStrategy, Action<Context, string, Exception> onCacheError = null)
+    public static CachePolicy<TResult> Cache<TResult>(ISyncCacheProvider cacheProvider, TimeSpan ttl, Func<Context, string> cacheKeyStrategy, Action<Context, string, Exception>? onCacheError = null)
     {
         if (cacheProvider == null) throw new ArgumentNullException(nameof(cacheProvider));
 
@@ -114,7 +115,7 @@ public static CachePolicy<TResult> Cache<TResult>(ISyncCacheProvider cacheProvid
     /// <exception cref="ArgumentNullException">cacheProvider</exception>
     /// <exception cref="ArgumentNullException">ttlStrategy</exception>
     /// <exception cref="ArgumentNullException">cacheKeyStrategy</exception>
-    public static CachePolicy<TResult> Cache<TResult>(ISyncCacheProvider cacheProvider, ITtlStrategy ttlStrategy, Func<Context, string> cacheKeyStrategy, Action<Context, string, Exception> onCacheError = null)
+    public static CachePolicy<TResult> Cache<TResult>(ISyncCacheProvider cacheProvider, ITtlStrategy ttlStrategy, Func<Context, string> cacheKeyStrategy, Action<Context, string, Exception>? onCacheError = null)
     {
         if (cacheProvider == null) throw new ArgumentNullException(nameof(cacheProvider));
 
@@ -351,7 +352,7 @@ public static CachePolicy<TResult> Cache<TResult>(
     /// <param name="onCacheError">Delegate to call if an exception is thrown when attempting to get a value from or put a value into the cache, passing the execution context, the cache key, and the exception.</param>
     /// <returns>The policy instance.</returns>
     /// <exception cref="ArgumentNullException">cacheProvider</exception>
-    public static CachePolicy<TResult> Cache<TResult>(ISyncCacheProvider<TResult> cacheProvider, TimeSpan ttl, Action<Context, string, Exception> onCacheError = null)
+    public static CachePolicy<TResult> Cache<TResult>(ISyncCacheProvider<TResult> cacheProvider, TimeSpan ttl, Action<Context, string, Exception>? onCacheError = null)
         => Cache<TResult>(cacheProvider, new RelativeTtl(ttl), DefaultCacheKeyStrategy.Instance.GetCacheKey, onCacheError);
 
     /// <summary>
@@ -366,7 +367,7 @@ public static CachePolicy<TResult> Cache<TResult>(ISyncCacheProvider<TResult> ca
     /// <returns>The policy instance.</returns>
     /// <exception cref="ArgumentNullException">cacheProvider</exception>
     /// <exception cref="ArgumentNullException">ttlStrategy</exception>
-    public static CachePolicy<TResult> Cache<TResult>(ISyncCacheProvider<TResult> cacheProvider, ITtlStrategy ttlStrategy, Action<Context, string, Exception> onCacheError = null)
+    public static CachePolicy<TResult> Cache<TResult>(ISyncCacheProvider<TResult> cacheProvider, ITtlStrategy ttlStrategy, Action<Context, string, Exception>? onCacheError = null)
         => Cache<TResult>(cacheProvider, ttlStrategy, DefaultCacheKeyStrategy.Instance.GetCacheKey, onCacheError);
 
     /// <summary>
@@ -381,7 +382,7 @@ public static CachePolicy<TResult> Cache<TResult>(ISyncCacheProvider<TResult> ca
     /// <returns>The policy instance.</returns>
     /// <exception cref="ArgumentNullException">cacheProvider</exception>
     /// <exception cref="ArgumentNullException">ttlStrategy</exception>
-    public static CachePolicy<TResult> Cache<TResult>(ISyncCacheProvider<TResult> cacheProvider, ITtlStrategy<TResult> ttlStrategy, Action<Context, string, Exception> onCacheError = null)
+    public static CachePolicy<TResult> Cache<TResult>(ISyncCacheProvider<TResult> cacheProvider, ITtlStrategy<TResult> ttlStrategy, Action<Context, string, Exception>? onCacheError = null)
         => Cache<TResult>(cacheProvider, ttlStrategy, DefaultCacheKeyStrategy.Instance.GetCacheKey, onCacheError);
 
     /// <summary>
@@ -397,7 +398,7 @@ public static CachePolicy<TResult> Cache<TResult>(ISyncCacheProvider<TResult> ca
     /// <returns>The policy instance.</returns>
     /// <exception cref="ArgumentNullException">cacheProvider</exception>
     /// <exception cref="ArgumentNullException">cacheKeyStrategy</exception>
-    public static CachePolicy<TResult> Cache<TResult>(ISyncCacheProvider<TResult> cacheProvider, TimeSpan ttl, ICacheKeyStrategy cacheKeyStrategy, Action<Context, string, Exception> onCacheError = null)
+    public static CachePolicy<TResult> Cache<TResult>(ISyncCacheProvider<TResult> cacheProvider, TimeSpan ttl, ICacheKeyStrategy cacheKeyStrategy, Action<Context, string, Exception>? onCacheError = null)
         => Cache<TResult>(cacheProvider, new RelativeTtl(ttl), cacheKeyStrategy.GetCacheKey, onCacheError);
 
     /// <summary>
@@ -414,7 +415,7 @@ public static CachePolicy<TResult> Cache<TResult>(ISyncCacheProvider<TResult> ca
     /// <exception cref="ArgumentNullException">cacheProvider</exception>
     /// <exception cref="ArgumentNullException">ttlStrategy</exception>
     /// <exception cref="ArgumentNullException">cacheKeyStrategy</exception>
-    public static CachePolicy<TResult> Cache<TResult>(ISyncCacheProvider<TResult> cacheProvider, ITtlStrategy ttlStrategy, ICacheKeyStrategy cacheKeyStrategy, Action<Context, string, Exception> onCacheError = null)
+    public static CachePolicy<TResult> Cache<TResult>(ISyncCacheProvider<TResult> cacheProvider, ITtlStrategy ttlStrategy, ICacheKeyStrategy cacheKeyStrategy, Action<Context, string, Exception>? onCacheError = null)
     {
         onCacheError = onCacheError ?? ((_, _, _) => { });
 
@@ -438,7 +439,7 @@ public static CachePolicy<TResult> Cache<TResult>(ISyncCacheProvider<TResult> ca
     /// <exception cref="ArgumentNullException">cacheProvider</exception>
     /// <exception cref="ArgumentNullException">ttlStrategy</exception>
     /// <exception cref="ArgumentNullException">cacheKeyStrategy</exception>
-    public static CachePolicy<TResult> Cache<TResult>(ISyncCacheProvider<TResult> cacheProvider, ITtlStrategy<TResult> ttlStrategy, ICacheKeyStrategy cacheKeyStrategy, Action<Context, string, Exception> onCacheError = null)
+    public static CachePolicy<TResult> Cache<TResult>(ISyncCacheProvider<TResult> cacheProvider, ITtlStrategy<TResult> ttlStrategy, ICacheKeyStrategy cacheKeyStrategy, Action<Context, string, Exception>? onCacheError = null)
     {
         onCacheError = onCacheError ?? ((_, _, _) => { });
 
@@ -461,7 +462,7 @@ public static CachePolicy<TResult> Cache<TResult>(ISyncCacheProvider<TResult> ca
     /// <returns>The policy instance.</returns>
     /// <exception cref="ArgumentNullException">cacheProvider</exception>
     /// <exception cref="ArgumentNullException">cacheKeyStrategy</exception>
-    public static CachePolicy<TResult> Cache<TResult>(ISyncCacheProvider<TResult> cacheProvider, TimeSpan ttl, Func<Context, string> cacheKeyStrategy, Action<Context, string, Exception> onCacheError = null)
+    public static CachePolicy<TResult> Cache<TResult>(ISyncCacheProvider<TResult> cacheProvider, TimeSpan ttl, Func<Context, string> cacheKeyStrategy, Action<Context, string, Exception>? onCacheError = null)
         => Cache<TResult>(cacheProvider, new RelativeTtl(ttl), cacheKeyStrategy, onCacheError);
 
     /// <summary>
@@ -478,7 +479,7 @@ public static CachePolicy<TResult> Cache<TResult>(ISyncCacheProvider<TResult> ca
     /// <exception cref="ArgumentNullException">cacheProvider</exception>
     /// <exception cref="ArgumentNullException">ttlStrategy</exception>
     /// <exception cref="ArgumentNullException">cacheKeyStrategy</exception>
-    public static CachePolicy<TResult> Cache<TResult>(ISyncCacheProvider<TResult> cacheProvider, ITtlStrategy ttlStrategy, Func<Context, string> cacheKeyStrategy, Action<Context, string, Exception> onCacheError = null)
+    public static CachePolicy<TResult> Cache<TResult>(ISyncCacheProvider<TResult> cacheProvider, ITtlStrategy ttlStrategy, Func<Context, string> cacheKeyStrategy, Action<Context, string, Exception>? onCacheError = null)
     {
         onCacheError = onCacheError ?? ((_, _, _) => { });
 
@@ -502,7 +503,7 @@ public static CachePolicy<TResult> Cache<TResult>(ISyncCacheProvider<TResult> ca
     /// <exception cref="ArgumentNullException">cacheProvider</exception>
     /// <exception cref="ArgumentNullException">ttlStrategy</exception>
     /// <exception cref="ArgumentNullException">cacheKeyStrategy</exception>
-    public static CachePolicy<TResult> Cache<TResult>(ISyncCacheProvider<TResult> cacheProvider, ITtlStrategy<TResult> ttlStrategy, Func<Context, string> cacheKeyStrategy, Action<Context, string, Exception> onCacheError = null)
+    public static CachePolicy<TResult> Cache<TResult>(ISyncCacheProvider<TResult> cacheProvider, ITtlStrategy<TResult> ttlStrategy, Func<Context, string> cacheKeyStrategy, Action<Context, string, Exception>? onCacheError = null)
     {
         onCacheError = onCacheError ?? ((_, _, _) => { });
 
diff --git a/src/Polly/Retry/RetryEngine.cs b/src/Polly/Retry/RetryEngine.cs
index 1773d0890dd..527abe0972d 100644
--- a/src/Polly/Retry/RetryEngine.cs
+++ b/src/Polly/Retry/RetryEngine.cs
@@ -1,4 +1,5 @@
-namespace Polly.Retry;
+#nullable enable
+namespace Polly.Retry;
 
 internal static class RetryEngine
 {
@@ -10,11 +11,11 @@ internal static TResult Implementation<TResult>(
         ResultPredicates<TResult> shouldRetryResultPredicates,
         Action<DelegateResult<TResult>, TimeSpan, int, Context> onRetry,
         int permittedRetryCount = Int32.MaxValue,
-        IEnumerable<TimeSpan> sleepDurationsEnumerable = null,
-        Func<int, DelegateResult<TResult>, Context, TimeSpan> sleepDurationProvider = null)
+        IEnumerable<TimeSpan>? sleepDurationsEnumerable = null,
+        Func<int, DelegateResult<TResult>, Context, TimeSpan>? sleepDurationProvider = null)
     {
         int tryCount = 0;
-        IEnumerator<TimeSpan> sleepDurationsEnumerator = sleepDurationsEnumerable?.GetEnumerator();
+        IEnumerator<TimeSpan>? sleepDurationsEnumerator = sleepDurationsEnumerable?.GetEnumerator();
 
         try
         {
diff --git a/src/Polly/Retry/RetryPolicy.cs b/src/Polly/Retry/RetryPolicy.cs
index 8f043d18f65..e97b3535c7b 100644
--- a/src/Polly/Retry/RetryPolicy.cs
+++ b/src/Polly/Retry/RetryPolicy.cs
@@ -1,4 +1,5 @@
-namespace Polly.Retry;
+#nullable enable
+namespace Polly.Retry;
 
 /// <summary>
 /// A retry policy that can be applied to synchronous delegates.
@@ -7,15 +8,15 @@ public class RetryPolicy : Policy, IRetryPolicy
 {
     private readonly Action<Exception, TimeSpan, int, Context> _onRetry;
     private readonly int _permittedRetryCount;
-    private readonly IEnumerable<TimeSpan> _sleepDurationsEnumerable;
-    private readonly Func<int, Exception, Context, TimeSpan> _sleepDurationProvider;
+    private readonly IEnumerable<TimeSpan>? _sleepDurationsEnumerable;
+    private readonly Func<int, Exception, Context, TimeSpan>? _sleepDurationProvider;
 
     internal RetryPolicy(
         PolicyBuilder policyBuilder,
         Action<Exception, TimeSpan, int, Context> onRetry,
         int permittedRetryCount = Int32.MaxValue,
-        IEnumerable<TimeSpan> sleepDurationsEnumerable = null,
-        Func<int, Exception, Context, TimeSpan> sleepDurationProvider = null
+        IEnumerable<TimeSpan>? sleepDurationsEnumerable = null,
+        Func<int, Exception, Context, TimeSpan>? sleepDurationProvider = null
         )
         : base(policyBuilder)
     {
@@ -38,7 +39,7 @@ protected override TResult Implementation<TResult>(Func<Context, CancellationTok
                 _sleepDurationsEnumerable,
                 _sleepDurationProvider != null
                     ? (retryCount, outcome, ctx) => _sleepDurationProvider(retryCount, outcome.Exception, ctx)
-                    : (Func<int, DelegateResult<TResult>, Context, TimeSpan>)null
+                    : (Func<int, DelegateResult<TResult>, Context, TimeSpan>?)null
             );
 }
 
@@ -49,15 +50,15 @@ public class RetryPolicy<TResult> : Policy<TResult>, IRetryPolicy<TResult>
 {
     private readonly Action<DelegateResult<TResult>, TimeSpan, int, Context> _onRetry;
     private readonly int _permittedRetryCount;
-    private readonly IEnumerable<TimeSpan> _sleepDurationsEnumerable;
-    private readonly Func<int, DelegateResult<TResult>, Context, TimeSpan> _sleepDurationProvider;
+    private readonly IEnumerable<TimeSpan>? _sleepDurationsEnumerable;
+    private readonly Func<int, DelegateResult<TResult>, Context, TimeSpan>? _sleepDurationProvider;
 
     internal RetryPolicy(
         PolicyBuilder<TResult> policyBuilder,
         Action<DelegateResult<TResult>, TimeSpan, int, Context> onRetry,
         int permittedRetryCount = Int32.MaxValue,
-        IEnumerable<TimeSpan> sleepDurationsEnumerable = null,
-        Func<int, DelegateResult<TResult>, Context, TimeSpan> sleepDurationProvider = null
+        IEnumerable<TimeSpan>? sleepDurationsEnumerable = null,
+        Func<int, DelegateResult<TResult>, Context, TimeSpan>? sleepDurationProvider = null
     )
         : base(policyBuilder)
     {