diff --git a/js/src/jsarray.cpp b/js/src/jsarray.cpp
index 80bba66edda..b7310018f0f 100644
--- a/js/src/jsarray.cpp
+++ b/js/src/jsarray.cpp
@@ -1275,11 +1275,24 @@ JSClass js_SlowArrayClass = {
 JSBool
 js_MakeArraySlow(JSContext *cx, JSObject *obj)
 {
-    JS_ASSERT(OBJ_GET_CLASS(cx, obj) == &js_ArrayClass);
+    JS_ASSERT(obj->getClass() == &js_ArrayClass);
 
-    /* Create a native scope. */
-    JSScope *scope = JSScope::create(cx, &js_SlowArrayObjectOps,
-                                     &js_SlowArrayClass, obj);
+    /*
+     * Create a native scope. All slow arrays other than Array.prototype get
+     * the same initial shape.
+     */
+    uint32 emptyShape;
+    JSObject *arrayProto = obj->getProto();
+    if (arrayProto->getClass() == &js_ObjectClass) {
+        /* obj is Array.prototype. */
+        emptyShape = js_GenerateShape(cx, false);
+    } else {
+        JS_ASSERT(arrayProto->getClass() == &js_SlowArrayClass);
+        if (!OBJ_SCOPE(arrayProto)->getEmptyScopeShape(cx, &js_SlowArrayClass, &emptyShape))
+            return JS_FALSE;
+    }
+    JSScope *scope = JSScope::create(cx, &js_SlowArrayObjectOps, &js_SlowArrayClass, obj,
+                                     emptyShape);
     if (!scope)
         return JS_FALSE;
 
diff --git a/js/src/jsobj.cpp b/js/src/jsobj.cpp
index 1c38cdb7137..804fddaa051 100644
--- a/js/src/jsobj.cpp
+++ b/js/src/jsobj.cpp
@@ -2025,7 +2025,7 @@ InitScopeForObject(JSContext* cx, JSObject* obj, JSObject* proto, JSObjectOps* o
         if (!scope)
             goto bad;
     } else {
-        scope = JSScope::create(cx, ops, clasp, obj);
+        scope = JSScope::create(cx, ops, clasp, obj, js_GenerateShape(cx, false));
         if (!scope)
             goto bad;
 
diff --git a/js/src/jsscope.cpp b/js/src/jsscope.cpp
index 7f377fa63d4..c4964337234 100644
--- a/js/src/jsscope.cpp
+++ b/js/src/jsscope.cpp
@@ -99,7 +99,7 @@ js_GetMutableScope(JSContext *cx, JSObject *obj)
      * birth, and runtime clone of a block objects are never mutated.
      */
     JS_ASSERT(STOBJ_GET_CLASS(obj) != &js_BlockClass);
-    newscope = JSScope::create(cx, scope->map.ops, obj->getClass(), obj);
+    newscope = JSScope::create(cx, scope->map.ops, obj->getClass(), obj, scope->shape);
     if (!newscope)
         return NULL;
     JS_LOCK_SCOPE(cx, newscope);
@@ -132,9 +132,9 @@ js_GetMutableScope(JSContext *cx, JSObject *obj)
 #define SCOPE_TABLE_NBYTES(n)   ((n) * sizeof(JSScopeProperty *))
 
 void
-JSScope::initMinimal(JSContext *cx)
+JSScope::initMinimal(JSContext *cx, uint32 newShape)
 {
-    shape = js_GenerateShape(cx, false);
+    shape = newShape;
     emptyScope = NULL;
     hashShift = JS_DHASH_BITS - MIN_SCOPE_SIZE_LOG2;
     entryCount = removedCount = 0;
@@ -183,7 +183,7 @@ JSScope::createTable(JSContext *cx, bool report)
 }
 
 JSScope *
-JSScope::create(JSContext *cx, JSObjectOps *ops, JSClass *clasp, JSObject *obj)
+JSScope::create(JSContext *cx, JSObjectOps *ops, JSClass *clasp, JSObject *obj, uint32 shape)
 {
     JS_ASSERT(OPS_IS_NATIVE(ops));
     JS_ASSERT(obj);
@@ -198,7 +198,7 @@ JSScope::create(JSContext *cx, JSObjectOps *ops, JSClass *clasp, JSObject *obj)
     scope->freeslot = JSSLOT_FREE(clasp);
     scope->flags = cx->runtime->gcRegenShapesScopeFlag;
     js_LeaveTraceIfGlobalObject(cx, obj);
-    scope->initMinimal(cx);
+    scope->initMinimal(cx, shape);
 
 #ifdef JS_THREADSAFE
     js_InitTitle(cx, &scope->title);
@@ -227,7 +227,7 @@ JSScope::createEmptyScope(JSContext *cx, JSClass *clasp)
     scope->nrefs = 2;
     scope->freeslot = JSSLOT_FREE(clasp);
     scope->flags = OWN_SHAPE | cx->runtime->gcRegenShapesScopeFlag;
-    scope->initMinimal(cx);
+    scope->initMinimal(cx, js_GenerateShape(cx, false));
 
 #ifdef JS_THREADSAFE
     js_InitTitle(cx, &scope->title);
@@ -1559,7 +1559,21 @@ JSScope::clear(JSContext *cx)
         js_free(table);
     clearMiddleDelete();
     js_LeaveTraceIfGlobalObject(cx, object);
-    initMinimal(cx);
+
+    JSClass *clasp = object->getClass();
+    JSObject *proto = object->getProto();
+    uint32 newShape;
+    if (proto && clasp == proto->getClass()) {
+#ifdef DEBUG
+        bool ok =
+#endif
+        OBJ_SCOPE(proto)->getEmptyScopeShape(cx, clasp, &newShape);
+        JS_ASSERT(ok);
+    } else {
+        newShape = js_GenerateShape(cx, false);
+    }
+    initMinimal(cx, newShape);
+
     JS_ATOMIC_INCREMENT(&cx->runtime->propertyRemovals);
 }
 
diff --git a/js/src/jsscope.h b/js/src/jsscope.h
index 211b88c9014..6fc56a3ed4f 100644
--- a/js/src/jsscope.h
+++ b/js/src/jsscope.h
@@ -215,7 +215,7 @@ struct JSScope {
     JSScopeProperty *lastProp;          /* pointer to last property added */
 
   private:
-    void initMinimal(JSContext *cx);
+    void initMinimal(JSContext *cx, uint32 newShape);
     bool createTable(JSContext *cx, bool report);
     bool changeTable(JSContext *cx, int change);
     void reportReadOnlyScope(JSContext *cx);
@@ -226,7 +226,8 @@ struct JSScope {
 
   public:
     /* Create a mutable, owned, empty scope. */
-    static JSScope *create(JSContext *cx, JSObjectOps *ops, JSClass *clasp, JSObject *obj);
+    static JSScope *create(JSContext *cx, JSObjectOps *ops, JSClass *clasp, JSObject *obj,
+                           uint32 shape);
 
     static void destroy(JSContext *cx, JSScope *scope);
 
@@ -245,6 +246,19 @@ struct JSScope {
         return createEmptyScope(cx, clasp);
     }
 
+    bool getEmptyScopeShape(JSContext *cx, JSClass *clasp, uint32 *shapep) {
+        if (emptyScope) {
+            *shapep = emptyScope->shape;
+            return true;
+        }
+        JSScope *e = getEmptyScope(cx, clasp);
+        if (!e)
+            return false;
+        *shapep = e->shape;
+        e->drop(cx, NULL);
+        return true;
+    }
+
     inline void hold();
     inline bool drop(JSContext *cx, JSObject *obj);