diff --git a/core/src/main/kotlin/org/jetbrains/kotlinx/dataframe/api/sort.kt b/core/src/main/kotlin/org/jetbrains/kotlinx/dataframe/api/sort.kt
index 37a009dfc5..cdafcc1dab 100644
--- a/core/src/main/kotlin/org/jetbrains/kotlinx/dataframe/api/sort.kt
+++ b/core/src/main/kotlin/org/jetbrains/kotlinx/dataframe/api/sort.kt
@@ -33,6 +33,15 @@ public interface SortDsl<out T> : ColumnsSelectionDsl<T> {
     public fun <C> KProperty<C?>.nullsLast(flag: Boolean = true): ColumnSet<C?> = toColumnAccessor().nullsLast(flag)
 }
 
+/**
+ * [SortColumnsSelector] is used to express or select multiple columns to sort by, represented by [ColumnSet]`<C>`,
+ * using the context of [SortDsl]`<T>` as `this` and `it`.
+ *
+ * So:
+ * ```kotlin
+ * SortDsl<T>.(it: SortDsl<T>) -> ColumnSet<C>
+ * ```
+ */
 public typealias SortColumnsSelector<T, C> = Selector<SortDsl<T>, ColumnSet<C>>
 
 // region DataColumn
diff --git a/core/src/main/kotlin/org/jetbrains/kotlinx/dataframe/impl/DataFrameReceiver.kt b/core/src/main/kotlin/org/jetbrains/kotlinx/dataframe/impl/DataFrameReceiver.kt
index cc8ab39487..3345f87fe9 100644
--- a/core/src/main/kotlin/org/jetbrains/kotlinx/dataframe/impl/DataFrameReceiver.kt
+++ b/core/src/main/kotlin/org/jetbrains/kotlinx/dataframe/impl/DataFrameReceiver.kt
@@ -36,12 +36,17 @@ internal open class DataFrameReceiver<T>(
     private val unresolvedColumnsPolicy: UnresolvedColumnsPolicy
 ) : DataFrameReceiverBase<T>(source.unbox()), SingleColumn<DataRow<T>> {
 
-    private fun <R> DataColumn<R>?.check(path: ColumnPath): DataColumn<R>? =
+    private fun <R> DataColumn<R>?.check(path: ColumnPath): DataColumn<R> =
         when (this) {
             null -> when (unresolvedColumnsPolicy) {
-                UnresolvedColumnsPolicy.Create, UnresolvedColumnsPolicy.Skip -> MissingColumnGroup<Any>(path, this@DataFrameReceiver).asDataColumn().cast()
+                UnresolvedColumnsPolicy.Create, UnresolvedColumnsPolicy.Skip -> MissingColumnGroup<Any>(
+                    path,
+                    this@DataFrameReceiver
+                ).asDataColumn().cast()
+
                 UnresolvedColumnsPolicy.Fail -> error("Column $path not found")
             }
+
             is MissingDataColumn -> this
             is ColumnGroup<*> -> ColumnGroupWithParent(null, this).asDataColumn().cast()
             else -> this
diff --git a/core/src/main/kotlin/org/jetbrains/kotlinx/dataframe/impl/api/sort.kt b/core/src/main/kotlin/org/jetbrains/kotlinx/dataframe/impl/api/sort.kt
index f5d9b75ca7..c0a7ae71db 100644
--- a/core/src/main/kotlin/org/jetbrains/kotlinx/dataframe/impl/api/sort.kt
+++ b/core/src/main/kotlin/org/jetbrains/kotlinx/dataframe/impl/api/sort.kt
@@ -16,22 +16,29 @@ import org.jetbrains.kotlinx.dataframe.columns.UnresolvedColumnsPolicy
 import org.jetbrains.kotlinx.dataframe.columns.ValueColumn
 import org.jetbrains.kotlinx.dataframe.impl.columns.addPath
 import org.jetbrains.kotlinx.dataframe.impl.columns.assertIsComparable
+import org.jetbrains.kotlinx.dataframe.impl.columns.missing.MissingColumnGroup
 import org.jetbrains.kotlinx.dataframe.impl.columns.resolve
 import org.jetbrains.kotlinx.dataframe.impl.columns.toColumns
 import org.jetbrains.kotlinx.dataframe.kind
 import org.jetbrains.kotlinx.dataframe.nrow
 
-internal fun <T, G> GroupBy<T, G>.sortByImpl(columns: SortColumnsSelector<G, *>): GroupBy<T, G> {
-    return toDataFrame()
+@Suppress("UNCHECKED_CAST", "RemoveExplicitTypeArguments")
+internal fun <T, G> GroupBy<T, G>.sortByImpl(columns: SortColumnsSelector<G, *>): GroupBy<T, G> =
+    toDataFrame()
+
+        // sort the individual groups by the columns specified
         .update { groups }
         .with { it.sortByImpl(UnresolvedColumnsPolicy.Skip, columns) }
+
+        // sort the groups by the columns specified (must be either be the keys column or "groups")
+        // will do nothing if the columns specified are not the keys column or "groups"
         .sortByImpl(UnresolvedColumnsPolicy.Skip, columns as SortColumnsSelector<T, *>)
-        .asGroupBy { it.getFrameColumn(groups.name()).castFrameColumn() }
-}
+
+        .asGroupBy { it.getFrameColumn(groups.name()).castFrameColumn<G>() }
 
 internal fun <T, C> DataFrame<T>.sortByImpl(
     unresolvedColumnsPolicy: UnresolvedColumnsPolicy = UnresolvedColumnsPolicy.Fail,
-    columns: SortColumnsSelector<T, C>
+    columns: SortColumnsSelector<T, C>,
 ): DataFrame<T> {
     val sortColumns = getSortColumns(columns, unresolvedColumnsPolicy)
     if (sortColumns.isEmpty()) return this
@@ -61,9 +68,10 @@ internal fun AnyCol.createComparator(nullsLast: Boolean): java.util.Comparator<I
 
 internal fun <T, C> DataFrame<T>.getSortColumns(
     columns: SortColumnsSelector<T, C>,
-    unresolvedColumnsPolicy: UnresolvedColumnsPolicy
-): List<SortColumnDescriptor<*>> {
-    return columns.toColumns().resolve(this, unresolvedColumnsPolicy)
+    unresolvedColumnsPolicy: UnresolvedColumnsPolicy,
+): List<SortColumnDescriptor<*>> =
+    columns.toColumns().resolve(this, unresolvedColumnsPolicy)
+        .filterNot { it.data is MissingColumnGroup<*> } // can appear using [DataColumn<R>?.check] with UnresolvedColumnsPolicy.Skip
         .map {
             when (val col = it.data) {
                 is SortColumnDescriptor<*> -> col
@@ -71,7 +79,6 @@ internal fun <T, C> DataFrame<T>.getSortColumns(
                 else -> throw IllegalStateException("Can not use ${col.kind} as sort column")
             }
         }
-}
 
 internal enum class SortFlag { Reversed, NullsLast }
 
@@ -86,12 +93,14 @@ internal fun <C> ColumnWithPath<C>.addFlag(flag: SortFlag): ColumnWithPath<C> {
                 SortFlag.NullsLast -> SortColumnDescriptor(col.column, col.direction, true)
             }
         }
+
         is ValueColumn -> {
             when (flag) {
                 SortFlag.Reversed -> SortColumnDescriptor(col, SortDirection.Desc)
                 SortFlag.NullsLast -> SortColumnDescriptor(col, SortDirection.Asc, true)
             }
         }
+
         else -> throw IllegalArgumentException("Can not apply sort flag to column kind ${col.kind}")
     }.addPath(path)
 }
@@ -103,7 +112,7 @@ internal class ColumnsWithSortFlag<C>(val column: ColumnSet<C>, val flag: SortFl
 internal class SortColumnDescriptor<C>(
     val column: ValueColumn<C>,
     val direction: SortDirection = SortDirection.Asc,
-    val nullsLast: Boolean = false
+    val nullsLast: Boolean = false,
 ) : ValueColumn<C> by column
 
 internal enum class SortDirection { Asc, Desc }
diff --git a/core/src/test/kotlin/org/jetbrains/kotlinx/dataframe/api/sortGroupedDataframe.kt b/core/src/test/kotlin/org/jetbrains/kotlinx/dataframe/api/sortGroupedDataframe.kt
new file mode 100644
index 0000000000..76d7b07edb
--- /dev/null
+++ b/core/src/test/kotlin/org/jetbrains/kotlinx/dataframe/api/sortGroupedDataframe.kt
@@ -0,0 +1,72 @@
+package org.jetbrains.kotlinx.dataframe.api
+
+import io.kotest.matchers.shouldBe
+import org.jetbrains.kotlinx.dataframe.DataFrame
+import org.jetbrains.kotlinx.dataframe.alsoDebug
+import org.jetbrains.kotlinx.dataframe.io.read
+import org.junit.Test
+
+class SortGroupedDataframeTests {
+
+    @Test
+    fun `Sorted grouped iris dataset`() {
+        val irisData = DataFrame.read("src/test/resources/irisDataset.csv")
+        irisData.alsoDebug()
+
+        irisData.groupBy("variety").let {
+            it.sortBy("petal.length").toString() shouldBe
+                it.sortBy { it["petal.length"] }.toString()
+        }
+    }
+
+    enum class State {
+        Idle, Productive, Maintenance
+    }
+
+    @Test
+    fun test4() {
+        class Event(val toolId: String, val state: State, val timestamp: Long)
+
+        val tool1 = "tool_1"
+        val tool2 = "tool_2"
+        val tool3 = "tool_3"
+
+        val events = listOf(
+            Event(tool1, State.Idle, 0),
+            Event(tool1, State.Productive, 5),
+            Event(tool2, State.Idle, 0),
+            Event(tool2, State.Maintenance, 10),
+            Event(tool2, State.Idle, 20),
+            Event(tool3, State.Idle, 0),
+            Event(tool3, State.Productive, 25),
+        ).toDataFrame()
+
+        val lastTimestamp = events.maxOf { getValue<Long>("timestamp") }
+        val groupBy = events
+            .groupBy("toolId")
+            .sortBy("timestamp")
+            .add("stateDuration") {
+                (next()?.getValue("timestamp") ?: lastTimestamp) - getValue<Long>("timestamp")
+            }
+
+        groupBy.toDataFrame().alsoDebug()
+        groupBy.schema().print()
+        groupBy.keys.print()
+        groupBy.keys[0].print()
+
+        val df1 = groupBy.updateGroups {
+            val missingValues = State.values().asList().toDataFrame {
+                "state" from { it }
+            }
+
+            val df = it
+                .fullJoin(missingValues, "state")
+                .fillNulls("stateDuration")
+                .with { 100L }
+
+            df.groupBy("state").sumFor("stateDuration")
+        }
+
+        df1.toDataFrame().alsoDebug().isNotEmpty() shouldBe true
+    }
+}
diff --git a/core/src/test/resources/irisDataset.csv b/core/src/test/resources/irisDataset.csv
new file mode 100644
index 0000000000..bf14e161bf
--- /dev/null
+++ b/core/src/test/resources/irisDataset.csv
@@ -0,0 +1,151 @@
+"sepal.length","sepal.width","petal.length","petal.width","variety"
+5.1,3.5,1.4,.2,"Setosa"
+4.9,3,1.4,.2,"Setosa"
+4.7,3.2,1.3,.2,"Setosa"
+4.6,3.1,1.5,.2,"Setosa"
+5,3.6,1.4,.2,"Setosa"
+5.4,3.9,1.7,.4,"Setosa"
+4.6,3.4,1.4,.3,"Setosa"
+5,3.4,1.5,.2,"Setosa"
+4.4,2.9,1.4,.2,"Setosa"
+4.9,3.1,1.5,.1,"Setosa"
+5.4,3.7,1.5,.2,"Setosa"
+4.8,3.4,1.6,.2,"Setosa"
+4.8,3,1.4,.1,"Setosa"
+4.3,3,1.1,.1,"Setosa"
+5.8,4,1.2,.2,"Setosa"
+5.7,4.4,1.5,.4,"Setosa"
+5.4,3.9,1.3,.4,"Setosa"
+5.1,3.5,1.4,.3,"Setosa"
+5.7,3.8,1.7,.3,"Setosa"
+5.1,3.8,1.5,.3,"Setosa"
+5.4,3.4,1.7,.2,"Setosa"
+5.1,3.7,1.5,.4,"Setosa"
+4.6,3.6,1,.2,"Setosa"
+5.1,3.3,1.7,.5,"Setosa"
+4.8,3.4,1.9,.2,"Setosa"
+5,3,1.6,.2,"Setosa"
+5,3.4,1.6,.4,"Setosa"
+5.2,3.5,1.5,.2,"Setosa"
+5.2,3.4,1.4,.2,"Setosa"
+4.7,3.2,1.6,.2,"Setosa"
+4.8,3.1,1.6,.2,"Setosa"
+5.4,3.4,1.5,.4,"Setosa"
+5.2,4.1,1.5,.1,"Setosa"
+5.5,4.2,1.4,.2,"Setosa"
+4.9,3.1,1.5,.2,"Setosa"
+5,3.2,1.2,.2,"Setosa"
+5.5,3.5,1.3,.2,"Setosa"
+4.9,3.6,1.4,.1,"Setosa"
+4.4,3,1.3,.2,"Setosa"
+5.1,3.4,1.5,.2,"Setosa"
+5,3.5,1.3,.3,"Setosa"
+4.5,2.3,1.3,.3,"Setosa"
+4.4,3.2,1.3,.2,"Setosa"
+5,3.5,1.6,.6,"Setosa"
+5.1,3.8,1.9,.4,"Setosa"
+4.8,3,1.4,.3,"Setosa"
+5.1,3.8,1.6,.2,"Setosa"
+4.6,3.2,1.4,.2,"Setosa"
+5.3,3.7,1.5,.2,"Setosa"
+5,3.3,1.4,.2,"Setosa"
+7,3.2,4.7,1.4,"Versicolor"
+6.4,3.2,4.5,1.5,"Versicolor"
+6.9,3.1,4.9,1.5,"Versicolor"
+5.5,2.3,4,1.3,"Versicolor"
+6.5,2.8,4.6,1.5,"Versicolor"
+5.7,2.8,4.5,1.3,"Versicolor"
+6.3,3.3,4.7,1.6,"Versicolor"
+4.9,2.4,3.3,1,"Versicolor"
+6.6,2.9,4.6,1.3,"Versicolor"
+5.2,2.7,3.9,1.4,"Versicolor"
+5,2,3.5,1,"Versicolor"
+5.9,3,4.2,1.5,"Versicolor"
+6,2.2,4,1,"Versicolor"
+6.1,2.9,4.7,1.4,"Versicolor"
+5.6,2.9,3.6,1.3,"Versicolor"
+6.7,3.1,4.4,1.4,"Versicolor"
+5.6,3,4.5,1.5,"Versicolor"
+5.8,2.7,4.1,1,"Versicolor"
+6.2,2.2,4.5,1.5,"Versicolor"
+5.6,2.5,3.9,1.1,"Versicolor"
+5.9,3.2,4.8,1.8,"Versicolor"
+6.1,2.8,4,1.3,"Versicolor"
+6.3,2.5,4.9,1.5,"Versicolor"
+6.1,2.8,4.7,1.2,"Versicolor"
+6.4,2.9,4.3,1.3,"Versicolor"
+6.6,3,4.4,1.4,"Versicolor"
+6.8,2.8,4.8,1.4,"Versicolor"
+6.7,3,5,1.7,"Versicolor"
+6,2.9,4.5,1.5,"Versicolor"
+5.7,2.6,3.5,1,"Versicolor"
+5.5,2.4,3.8,1.1,"Versicolor"
+5.5,2.4,3.7,1,"Versicolor"
+5.8,2.7,3.9,1.2,"Versicolor"
+6,2.7,5.1,1.6,"Versicolor"
+5.4,3,4.5,1.5,"Versicolor"
+6,3.4,4.5,1.6,"Versicolor"
+6.7,3.1,4.7,1.5,"Versicolor"
+6.3,2.3,4.4,1.3,"Versicolor"
+5.6,3,4.1,1.3,"Versicolor"
+5.5,2.5,4,1.3,"Versicolor"
+5.5,2.6,4.4,1.2,"Versicolor"
+6.1,3,4.6,1.4,"Versicolor"
+5.8,2.6,4,1.2,"Versicolor"
+5,2.3,3.3,1,"Versicolor"
+5.6,2.7,4.2,1.3,"Versicolor"
+5.7,3,4.2,1.2,"Versicolor"
+5.7,2.9,4.2,1.3,"Versicolor"
+6.2,2.9,4.3,1.3,"Versicolor"
+5.1,2.5,3,1.1,"Versicolor"
+5.7,2.8,4.1,1.3,"Versicolor"
+6.3,3.3,6,2.5,"Virginica"
+5.8,2.7,5.1,1.9,"Virginica"
+7.1,3,5.9,2.1,"Virginica"
+6.3,2.9,5.6,1.8,"Virginica"
+6.5,3,5.8,2.2,"Virginica"
+7.6,3,6.6,2.1,"Virginica"
+4.9,2.5,4.5,1.7,"Virginica"
+7.3,2.9,6.3,1.8,"Virginica"
+6.7,2.5,5.8,1.8,"Virginica"
+7.2,3.6,6.1,2.5,"Virginica"
+6.5,3.2,5.1,2,"Virginica"
+6.4,2.7,5.3,1.9,"Virginica"
+6.8,3,5.5,2.1,"Virginica"
+5.7,2.5,5,2,"Virginica"
+5.8,2.8,5.1,2.4,"Virginica"
+6.4,3.2,5.3,2.3,"Virginica"
+6.5,3,5.5,1.8,"Virginica"
+7.7,3.8,6.7,2.2,"Virginica"
+7.7,2.6,6.9,2.3,"Virginica"
+6,2.2,5,1.5,"Virginica"
+6.9,3.2,5.7,2.3,"Virginica"
+5.6,2.8,4.9,2,"Virginica"
+7.7,2.8,6.7,2,"Virginica"
+6.3,2.7,4.9,1.8,"Virginica"
+6.7,3.3,5.7,2.1,"Virginica"
+7.2,3.2,6,1.8,"Virginica"
+6.2,2.8,4.8,1.8,"Virginica"
+6.1,3,4.9,1.8,"Virginica"
+6.4,2.8,5.6,2.1,"Virginica"
+7.2,3,5.8,1.6,"Virginica"
+7.4,2.8,6.1,1.9,"Virginica"
+7.9,3.8,6.4,2,"Virginica"
+6.4,2.8,5.6,2.2,"Virginica"
+6.3,2.8,5.1,1.5,"Virginica"
+6.1,2.6,5.6,1.4,"Virginica"
+7.7,3,6.1,2.3,"Virginica"
+6.3,3.4,5.6,2.4,"Virginica"
+6.4,3.1,5.5,1.8,"Virginica"
+6,3,4.8,1.8,"Virginica"
+6.9,3.1,5.4,2.1,"Virginica"
+6.7,3.1,5.6,2.4,"Virginica"
+6.9,3.1,5.1,2.3,"Virginica"
+5.8,2.7,5.1,1.9,"Virginica"
+6.8,3.2,5.9,2.3,"Virginica"
+6.7,3.3,5.7,2.5,"Virginica"
+6.7,3,5.2,2.3,"Virginica"
+6.3,2.5,5,1.9,"Virginica"
+6.5,3,5.2,2,"Virginica"
+6.2,3.4,5.4,2.3,"Virginica"
+5.9,3,5.1,1.8,"Virginica"