diff --git a/lib/puppet/type/node_group.rb b/lib/puppet/type/node_group.rb
index d798e0d..8e5176c 100644
--- a/lib/puppet/type/node_group.rb
+++ b/lib/puppet/type/node_group.rb
@@ -123,7 +123,7 @@ def insync?(is)
     desc 'Environment for this group'
     defaultto :production
     validate do |value|
-      raise('Invalid environment name') unless value =~ (%r{\A[a-z0-9_]+\Z}) || (value == 'agent-specified')
+      raise('Invalid environment name') unless value =~ (%r{\A[a-zA-Z0-9_]+\Z}) || (value == 'agent-specified')
     end
   end
   newproperty(:classes) do
diff --git a/spec/unit/puppet/type/node_group_spec.rb b/spec/unit/puppet/type/node_group_spec.rb
index 90c17ad..831961e 100644
--- a/spec/unit/puppet/type/node_group_spec.rb
+++ b/spec/unit/puppet/type/node_group_spec.rb
@@ -20,6 +20,33 @@
     }.not_to raise_error
   end
 
+  it 'allows uppercase environment name without underscore' do
+    expect {
+      Puppet::Type.type(:node_group).new(
+        name: 'stubname',
+        environment: 'ENVIRONMENTNAME',
+      )
+    }.not_to raise_error
+  end
+
+  it 'allows uppercase environment name with an underscore' do
+    expect {
+      Puppet::Type.type(:node_group).new(
+        name: 'stubname',
+        environment: 'ENVIRONMENT_NAME',
+      )
+    }.not_to raise_error
+  end
+
+  it 'allows capitalized environment name with an underscore' do
+    expect {
+      Puppet::Type.type(:node_group).new(
+        name: 'stubname',
+        environment: 'Environment_name',
+      )
+    }.not_to raise_error
+  end
+
   it 'allows environment name with a number' do
     expect {
       Puppet::Type.type(:node_group).new(