--- a/js/src/vm/Interpreter.cpp
+++ b/js/src/vm/Interpreter.cpp
@@ -1487,16 +1487,19 @@ JS_STATIC_ASSERT(JSOP_IFNE == JSOP_IFEQ 
  * global.
  */
 static inline Value
 ComputeImplicitThis(JSObject* obj)
 {
     if (obj->is<GlobalObject>())
         return UndefinedValue();
 
+    if (obj->is<NonSyntacticVariablesObject>())
+        return UndefinedValue();
+
     if (IsCacheableEnvironment(obj))
         return UndefinedValue();
 
     // Debugger environments need special casing, as despite being
     // non-syntactic, they wrap syntactic environments and should not be
     // treated like other embedding-specific non-syntactic environments.
     if (obj->is<DebugEnvironmentProxy>())
         return ComputeImplicitThis(&obj->as<DebugEnvironmentProxy>().environment());