diff --git a/js/src/jsinterp.cpp b/js/src/jsinterp.cpp
index 61bd303acaca..882ec1c9ace1 100644
--- a/js/src/jsinterp.cpp
+++ b/js/src/jsinterp.cpp
@@ -3161,11 +3161,13 @@ BEGIN_CASE(JSOP_SETMETHOD)
              * on a prototype property that has a setter.
              */
             const Shape *shape = entry->prop;
-            JS_ASSERT_IF(shape->isDataDescriptor(), shape->writable());
-            JS_ASSERT_IF(shape->hasSlot(), entry->isOwnPropertyHit());
 
             if (entry->isOwnPropertyHit() ||
-                ((obj2 = obj->getProto()) && obj2->lastProperty() == entry->pshape)) {
+                ((obj2 = obj->getProto()) && obj2->lastProperty() == entry->pshape))
+            {
+                JS_ASSERT_IF(shape->isDataDescriptor(), shape->writable());
+                JS_ASSERT_IF(shape->hasSlot(), entry->isOwnPropertyHit());
+
 #ifdef DEBUG
                 if (entry->isOwnPropertyHit()) {
                     JS_ASSERT(obj->nativeContains(cx, *shape));
diff --git a/js/src/tests/ecma_5/Object/jstests.list b/js/src/tests/ecma_5/Object/jstests.list
index 320b23ce59b5..3bad7957e167 100644
--- a/js/src/tests/ecma_5/Object/jstests.list
+++ b/js/src/tests/ecma_5/Object/jstests.list
@@ -38,6 +38,7 @@ skip-if(!xulRuntime.shell) script 15.2.3.6-dictionary-redefinition-8-of-8.js # u
 script 15.2.3.6-define-over-method.js
 script mutation-prevention-methods.js
 script object-toString-01.js
+script proto-property-change-writability-set.js
 script vacuous-accessor-unqualified-name.js
 script add-property-non-extensible.js
 skip-if(!xulRuntime.shell) script freeze-global-eval-const.js # uses evalcx
diff --git a/js/src/tests/ecma_5/Object/proto-property-change-writability-set.js b/js/src/tests/ecma_5/Object/proto-property-change-writability-set.js
new file mode 100644
index 000000000000..f9b0eebd48e6
--- /dev/null
+++ b/js/src/tests/ecma_5/Object/proto-property-change-writability-set.js
@@ -0,0 +1,44 @@
+/*
+ * Any copyright is dedicated to the Public Domain.
+ * http://creativecommons.org/licenses/publicdomain/
+ * Contributors:
+ *   Gary Kwong
+ *   Jeff Walden
+ *   Jason Orendorff
+ */
+
+var accDesc = { set: function() {} };
+var dataDesc = { value: 3 };
+
+function f()
+{
+  constructor = {};
+}
+function g()
+{
+  constructor = {};
+}
+
+Object.defineProperty(Object.prototype, "constructor", accDesc);
+f();
+Object.defineProperty(Object.prototype, "constructor", dataDesc);
+assertEq(constructor, 3);
+f();
+assertEq(constructor, 3);
+g();
+assertEq(constructor, 3);
+
+
+
+var a = { p1: 1, p2: 2 };
+var b = Object.create(a);
+Object.defineProperty(a, "p1", {set: function () {}});
+for (var i = 0; i < 2; i++)
+{
+  b.p1 = {};
+  Object.defineProperty(a, "p1", {value: 3});
+}
+assertEq(b.p1, 3);
+assertEq(a.p1, 3);
+
+reportCompare(true, true);