diff --git a/src/objects.cc b/src/objects.cc
index f414c1596d..7495d6e709 100644
--- a/src/objects.cc
+++ b/src/objects.cc
@@ -2331,7 +2331,7 @@ void JSProxy::Fix() {
   Handle<JSProxy> self(this);
 
   isolate->factory()->BecomeJSObject(self);
-  ASSERT(IsJSObject());
+  ASSERT(self->IsJSObject());
   // TODO(rossberg): recognize function proxies.
 }
 
diff --git a/src/runtime.cc b/src/runtime.cc
index b4259c4a5c..2cf8aba18e 100644
--- a/src/runtime.cc
+++ b/src/runtime.cc
@@ -619,7 +619,7 @@ RUNTIME_FUNCTION(MaybeObject*, Runtime_Fix) {
   ASSERT(args.length() == 1);
   CONVERT_CHECKED(JSProxy, proxy, args[0]);
   proxy->Fix();
-  return proxy;
+  return isolate->heap()->undefined_value();
 }