diff --git a/engine/src/flutter/lib/web_ui/lib/src/engine/util.dart b/engine/src/flutter/lib/web_ui/lib/src/engine/util.dart
index 30771054efb..c410c8b2493 100644
--- a/engine/src/flutter/lib/web_ui/lib/src/engine/util.dart
+++ b/engine/src/flutter/lib/web_ui/lib/src/engine/util.dart
@@ -82,7 +82,7 @@ String float64ListToCssTransform(Float32List matrix) {
     return float64ListToCssTransform3d(matrix);
   } else {
     assert(transformKind == TransformKind.identity);
-    return null;
+    return 'none';
   }
 }