diff --git a/frontend_server/lib/server.dart b/frontend_server/lib/server.dart
index cdf958eed8d6b2b5d63928d562c5a1f05019c596..fed3766d7c142d6b55678c2d6812a92ad1d16d2e 100644
--- a/frontend_server/lib/server.dart
+++ b/frontend_server/lib/server.dart
@@ -42,7 +42,7 @@ class _FlutterFrontendCompiler implements frontend.CompilerInterface{
   }
 
   @override
-  Future<Null> rejectLastDelta() async {
+  Future<void> rejectLastDelta() async {
     return _compiler.rejectLastDelta();
   }