diff --git a/cty/function/stdlib/collection.go b/cty/function/stdlib/collection.go
index 279a20ee..a91821e9 100644
--- a/cty/function/stdlib/collection.go
+++ b/cty/function/stdlib/collection.go
@@ -525,6 +525,7 @@ func flattener(flattenList cty.Value) ([]cty.Value, []cty.ValueMarks, bool) {
 	if len(flattenListMarks) > 0 {
 		markses = append(markses, flattenListMarks)
 	}
+
 	if !flattenList.Length().IsKnown() {
 		// If we don't know the length of what we're flattening then we can't
 		// predict the length of our result yet either.
@@ -542,7 +543,7 @@ func flattener(flattenList cty.Value) ([]cty.Value, []cty.ValueMarks, bool) {
 			isKnown = false
 		}
 
-		if val.Type().IsListType() || val.Type().IsSetType() || val.Type().IsTupleType() {
+		if !val.IsNull() && (val.Type().IsListType() || val.Type().IsSetType() || val.Type().IsTupleType()) {
 			if !val.IsKnown() {
 				isKnown = false
 				_, unknownMarks := val.Unmark()
diff --git a/cty/function/stdlib/collection_test.go b/cty/function/stdlib/collection_test.go
index 0d5d5e83..ebcb6ea4 100644
--- a/cty/function/stdlib/collection_test.go
+++ b/cty/function/stdlib/collection_test.go
@@ -2064,6 +2064,110 @@ func TestFlatten(t *testing.T) {
 			cty.UnknownVal(cty.DynamicPseudoType),
 			"",
 		},
+		{
+			// null of an unknown type
+			cty.TupleVal([]cty.Value{
+				cty.NullVal(cty.DynamicPseudoType),
+				cty.True,
+			}),
+			cty.TupleVal([]cty.Value{
+				cty.NullVal(cty.DynamicPseudoType),
+				cty.True,
+			}),
+			"",
+		},
+		{
+			// null of a string type
+			cty.TupleVal([]cty.Value{
+				cty.NullVal(cty.String),
+				cty.True,
+			}),
+			cty.TupleVal([]cty.Value{
+				cty.NullVal(cty.String),
+				cty.True,
+			}),
+			"",
+		},
+		{
+			// null of a list type
+			cty.TupleVal([]cty.Value{
+				cty.NullVal(cty.List(cty.String)),
+				cty.True,
+			}),
+			cty.TupleVal([]cty.Value{
+				cty.NullVal(cty.List(cty.String)),
+				cty.True,
+			}),
+			"",
+		},
+		{
+			// null of a tuple type
+			cty.TupleVal([]cty.Value{
+				cty.NullVal(cty.EmptyTuple),
+				cty.True,
+			}),
+			cty.TupleVal([]cty.Value{
+				cty.NullVal(cty.EmptyTuple),
+				cty.True,
+			}),
+			"",
+		},
+		{
+			// nested null of an unknown type
+			cty.TupleVal([]cty.Value{
+				cty.TupleVal([]cty.Value{
+					cty.NullVal(cty.DynamicPseudoType),
+				}),
+				cty.True,
+			}),
+			cty.TupleVal([]cty.Value{
+				cty.NullVal(cty.DynamicPseudoType),
+				cty.True,
+			}),
+			"",
+		},
+		{
+			// nested null of a string type
+			cty.TupleVal([]cty.Value{
+				cty.TupleVal([]cty.Value{
+					cty.NullVal(cty.String),
+				}),
+				cty.True,
+			}),
+			cty.TupleVal([]cty.Value{
+				cty.NullVal(cty.String),
+				cty.True,
+			}),
+			"",
+		},
+		{
+			// nested null of a list type
+			cty.TupleVal([]cty.Value{
+				cty.TupleVal([]cty.Value{
+					cty.NullVal(cty.List(cty.String)),
+				}),
+				cty.True,
+			}),
+			cty.TupleVal([]cty.Value{
+				cty.NullVal(cty.List(cty.String)),
+				cty.True,
+			}),
+			"",
+		},
+		{
+			// nested null of a tuple type
+			cty.TupleVal([]cty.Value{
+				cty.TupleVal([]cty.Value{
+					cty.NullVal(cty.EmptyTuple),
+				}),
+				cty.True,
+			}),
+			cty.TupleVal([]cty.Value{
+				cty.NullVal(cty.EmptyTuple),
+				cty.True,
+			}),
+			"",
+		},
 	}
 
 	for _, test := range tests {