diff --git a/lib/ui/painting.dart b/lib/ui/painting.dart
index a8ebd7a632004..1ba835c509a0b 100644
--- a/lib/ui/painting.dart
+++ b/lib/ui/painting.dart
@@ -4689,7 +4689,7 @@ class Picture extends NativeFieldWrapperClass2 {
     );
   }
 
-  String _toImage(int width, int height, _Callback<_Image> callback) native 'Picture_toImage';
+  String? _toImage(int width, int height, _Callback<_Image> callback) native 'Picture_toImage';
 
   /// Release the resources used by this object. The object is no longer usable
   /// after this method is called.