diff --git a/src/objects/js-objects.cc b/src/objects/js-objects.cc
index 5ed726f3387..841eec0edf4 100644
--- a/src/objects/js-objects.cc
+++ b/src/objects/js-objects.cc
@@ -3867,6 +3867,7 @@ Maybe<bool> JSObject::PreventExtensionsWithTransition(
   }
 
   Handle<Map> old_map(object->map(), isolate);
+  old_map = Map::Update(isolate, old_map);
   TransitionsAccessor transitions(isolate, old_map);
   Map transition = transitions.SearchSpecial(*transition_marker);
   if (!transition.is_null()) {
diff --git a/test/mjsunit/regress-956426.js b/test/mjsunit/regress-956426.js
new file mode 100644
index 00000000000..93ccd7d36db
--- /dev/null
+++ b/test/mjsunit/regress-956426.js
@@ -0,0 +1,10 @@
+// Copyright 2019 the V8 project authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+// Flags: --allow-natives-syntax
+
+var b = { x: 0, y: 0, 0: '' };
+var a = { x: 0, y: 100000000000, 0: '' };
+Object.seal(b);
+b.x = '';