diff --git a/packages/flutter/lib/src/widgets/form.dart b/packages/flutter/lib/src/widgets/form.dart
index 4c3b120387..28cea99bcd 100644
--- a/packages/flutter/lib/src/widgets/form.dart
+++ b/packages/flutter/lib/src/widgets/form.dart
@@ -35,6 +35,7 @@ import 'will_pop_scope.dart';
 ///             if (value.isEmpty) {
 ///               return 'Please enter some text';
 ///             }
+///             return null;
 ///           },
 ///         ),
 ///         Padding(