diff --git a/firestore.rules b/firestore.rules
index 383afb0c..9203835f 100644
--- a/firestore.rules
+++ b/firestore.rules
@@ -34,7 +34,8 @@ service cloud.firestore {
     // must only be performed server-side is denied for all direct client writes.
 
     function isImmutable(fields) {
-      return !request.resource.data.diff(resource.data).affectedKeys().hasAny(fields);
+      return fields is list
+        && !request.resource.data.diff(resource.data).affectedKeys().hasAny(fields);
     }
 
     function isValidSwipeAction(action) {