diff --git a/src/objects/js-objects.cc b/src/objects/js-objects.cc
index 463bc42d4bd810e99b3f7f9ef5c2a50c735a73eb..87feeda6a70525dc2419394dc95be5bd7e85db4c 100644
--- a/src/objects/js-objects.cc
+++ b/src/objects/js-objects.cc
@@ -360,9 +360,7 @@ Maybe<bool> JSReceiver::SetOrCopyDataProperties(
       Nothing<bool>());
 
   if (!from->HasFastProperties() && target->HasFastProperties() &&
-      !target->IsJSGlobalProxy()) {
-    // JSProxy is always in slow-mode.
-    DCHECK(!target->IsJSProxy());
+      target->IsJSObject() && !target->IsJSGlobalProxy()) {
     // Convert to slow properties if we're guaranteed to overflow the number of
     // descriptors.
     int source_length;