diff --git a/js/src/gc/Heap.h b/js/src/gc/Heap.h
index 4f04ace8a66..aa866f5fc36 100644
--- a/js/src/gc/Heap.h
+++ b/js/src/gc/Heap.h
@@ -99,6 +99,7 @@ struct Cell
 
     inline JSRuntime *runtime() const;
     inline Zone *tenuredZone() const;
+    inline bool tenuredIsInsideZone(Zone *zone) const;
 
 #ifdef DEBUG
     inline bool isAligned() const;
@@ -994,6 +995,13 @@ Cell::tenuredZone() const
     return arenaHeader()->zone;
 }
 
+bool
+Cell::tenuredIsInsideZone(Zone *zone) const
+{
+    JS_ASSERT(isTenured());
+    return zone == arenaHeader()->zone;
+}
+
 #ifdef DEBUG
 bool
 Cell::isAligned() const
diff --git a/js/src/vm/String.h b/js/src/vm/String.h
index 0f7e3b7e57f..4247beec6a0 100644
--- a/js/src/vm/String.h
+++ b/js/src/vm/String.h
@@ -410,7 +410,7 @@ class JSString : public js::gc::Cell
     }
 
     JS::Zone *zone() const { return tenuredZone(); }
-    bool isInsideZone(JS::Zone *zone_) { return zone_ == zone(); }
+    bool isInsideZone(JS::Zone *zone_) { return tenuredIsInsideZone(zone_); }
     js::gc::AllocKind getAllocKind() const { return tenuredGetAllocKind(); }
 
     static inline void writeBarrierPre(JSString *str);