diff --git a/engine/src/flutter/lib/web_ui/dev/test_platform.dart b/engine/src/flutter/lib/web_ui/dev/test_platform.dart
index 330457c23b4..8e181eab37e 100644
--- a/engine/src/flutter/lib/web_ui/dev/test_platform.dart
+++ b/engine/src/flutter/lib/web_ui/dev/test_platform.dart
@@ -559,7 +559,7 @@ class BrowserManager {
     }).catchError((dynamic error, StackTrace stackTrace) {
       browser.close();
       if (completer.isCompleted) {
-        return;
+        return null;
       }
       completer.completeError(error, stackTrace);
     });