diff --git a/js/src/gc/GCInternals.h b/js/src/gc/GCInternals.h
index 713ef3af61..7d2340e36a 100644
--- a/js/src/gc/GCInternals.h
+++ b/js/src/gc/GCInternals.h
@@ -215,9 +215,11 @@ RelocationOverlay::forwardTo(Cell* cell)
     MOZ_ASSERT(!isForwarded());
     // The location of magic_ is important because it must never be valid to see
     // the value Relocated there in a GC thing that has not been moved.
-    static_assert(offsetof(RelocationOverlay, magic_) == offsetof(JSObject, group_) &&
-                  offsetof(RelocationOverlay, magic_) == offsetof(js::Shape, base_) &&
-                  offsetof(RelocationOverlay, magic_) == offsetof(JSString, d.u1.flags),
+    static_assert(offsetof(RelocationOverlay, magic_) == offsetof(JSObject, group_) + sizeof(uint32_t),
+                  "RelocationOverlay::magic_ is in the wrong location");
+    static_assert(offsetof(RelocationOverlay, magic_) == offsetof(js::Shape, base_) + sizeof(uint32_t),
+                  "RelocationOverlay::magic_ is in the wrong location");
+    static_assert(offsetof(RelocationOverlay, magic_) == offsetof(JSString, d.u1.length),
                   "RelocationOverlay::magic_ is in the wrong location");
     magic_ = Relocated;
     newLocation_ = cell;
diff --git a/js/src/jsfriendapi.h b/js/src/jsfriendapi.h
index 6f4c4bfa8d..3013802113 100644
--- a/js/src/jsfriendapi.h
+++ b/js/src/jsfriendapi.h
@@ -587,7 +587,7 @@ struct String
 {
     static const uint32_t INLINE_CHARS_BIT = JS_BIT(2);
     static const uint32_t LATIN1_CHARS_BIT = JS_BIT(6);
-    static const uint32_t ROPE_FLAGS       = 0;
+    static const uint32_t ROPE_FLAGS       = NON_ATOM_BIT;
     static const uint32_t TYPE_FLAGS_MASK  = JS_BIT(6) - 1;
     uint32_t flags;
     uint32_t length;
diff --git a/js/src/vm/String.h b/js/src/vm/String.h
index 309e0ceafc..1250959a3a 100644
--- a/js/src/vm/String.h
+++ b/js/src/vm/String.h
@@ -251,11 +251,11 @@ class JSString : public js::gc::TenuredCell
     static const uint32_t INLINE_CHARS_BIT       = JS_BIT(2);
     static const uint32_t ATOM_BIT               = JS_BIT(3);
 
-    static const uint32_t ROPE_FLAGS             = 0;
-    static const uint32_t DEPENDENT_FLAGS        = HAS_BASE_BIT;
-    static const uint32_t UNDEPENDED_FLAGS       = FLAT_BIT | HAS_BASE_BIT;
-    static const uint32_t EXTENSIBLE_FLAGS       = FLAT_BIT | JS_BIT(4);
-    static const uint32_t EXTERNAL_FLAGS         = JS_BIT(5);
+    static const uint32_t ROPE_FLAGS             = NON_ATOM_BIT;
+    static const uint32_t DEPENDENT_FLAGS        = NON_ATOM_BIT | HAS_BASE_BIT;
+    static const uint32_t UNDEPENDED_FLAGS       = NON_ATOM_BIT | FLAT_BIT | HAS_BASE_BIT;
+    static const uint32_t EXTENSIBLE_FLAGS       = NON_ATOM_BIT | FLAT_BIT | JS_BIT(4);
+    static const uint32_t EXTERNAL_FLAGS         = NON_ATOM_BIT | JS_BIT(5);
 
     static const uint32_t FAT_INLINE_MASK        = INLINE_CHARS_BIT | JS_BIT(4);
     static const uint32_t PERMANENT_ATOM_MASK    = ATOM_BIT | JS_BIT(5);
@@ -506,11 +506,12 @@ class JSString : public js::gc::TenuredCell
     static const JS::TraceKind TraceKind = JS::TraceKind::String;
 
     JS::Zone* zone() const {
-        if (isTenured()) {
+        if (isTenured())  {
             // Allow permanent atoms to be accessed across zones and runtimes.
             if (isPermanentAtom())
                 return zoneFromAnyThread();
             return asTenured().zone();
+		}
         return js::Nursery::getStringZone(this);
     }
 
@@ -1588,4 +1589,15 @@ JSAtom::asPropertyName()
     return static_cast<js::PropertyName*>(this);
 }
 
+namespace js {
+namespace gc {
+template<>
+inline JSString*
+Cell::as<JSString>() {
+    MOZ_ASSERT(is<JSString>());
+    return reinterpret_cast<JSString*>(this);
+}
+}
+}
+
 #endif /* vm_String_h */