diff --git a/cty/function/stdlib/collection.go b/cty/function/stdlib/collection.go
index ad2bb637..dbfd46d0 100644
--- a/cty/function/stdlib/collection.go
+++ b/cty/function/stdlib/collection.go
@@ -530,6 +530,12 @@ func flattener(flattenList cty.Value) ([]cty.Value, []cty.ValueMarks, bool) {
 		// predict the length of our result yet either.
 		return nil, markses, false
 	}
+	// Any dynamic types could result in more collection that need to be
+	// flattened, so the type cannot be known.
+	if flattenList.Type().HasDynamicTypes() {
+		return nil, markses, false
+	}
+
 	out := make([]cty.Value, 0)
 	isKnown := true
 	for it := flattenList.ElementIterator(); it.Next(); {
diff --git a/cty/function/stdlib/collection_test.go b/cty/function/stdlib/collection_test.go
index 5bca5f85..be155526 100644
--- a/cty/function/stdlib/collection_test.go
+++ b/cty/function/stdlib/collection_test.go
@@ -1928,6 +1928,59 @@ func TestFlatten(t *testing.T) {
 			cty.EmptyTupleVal.Mark("a"),
 			"",
 		},
+		{
+			cty.ListValEmpty(cty.Number),
+			cty.EmptyTupleVal,
+			"",
+		},
+		{
+			cty.ListVal([]cty.Value{
+				cty.DynamicVal,
+			}),
+			cty.DynamicVal,
+			"",
+		},
+		{
+			cty.TupleVal([]cty.Value{
+				cty.ListVal([]cty.Value{
+					cty.ListVal([]cty.Value{
+						cty.DynamicVal,
+					}),
+				}),
+				cty.ListVal([]cty.Value{
+					cty.ListVal([]cty.Value{
+						cty.DynamicVal,
+					}).Mark("marked"),
+				}),
+			}),
+			cty.DynamicVal,
+			"",
+		},
+		{
+			cty.TupleVal([]cty.Value{
+				cty.StringVal("a"),
+				cty.ListVal([]cty.Value{
+					cty.StringVal("b"),
+				}),
+				cty.TupleVal([]cty.Value{
+					cty.ListVal([]cty.Value{
+						cty.StringVal("c"),
+					}),
+					cty.ListVal([]cty.Value{
+						cty.StringVal("d"),
+						cty.StringVal("e"),
+					}),
+				}),
+			}),
+			cty.TupleVal([]cty.Value{
+				cty.StringVal("a"),
+				cty.StringVal("b"),
+				cty.StringVal("c"),
+				cty.StringVal("d"),
+				cty.StringVal("e"),
+			}),
+			"",
+		},
 	}
 
 	for _, test := range tests {