diff --git a/built_value/lib/built_value.dart b/built_value/lib/built_value.dart
index 99c5b705..55de5b55 100644
--- a/built_value/lib/built_value.dart
+++ b/built_value/lib/built_value.dart
@@ -18,7 +18,7 @@ abstract class Built<V extends Built<V, B>, B extends Builder<V, B>> {
   ///
   /// The implementation of this method will be generated for you by the
   /// built_value generator.
-  V rebuild(Function(B) updates);
+  V rebuild(void Function(B) updates);
 
   /// Converts the instance to a builder [B].
   ///