diff --git a/lib/scope.js b/lib/scope.js
index b15d72e4..b2a073e6 100644
--- a/lib/scope.js
+++ b/lib/scope.js
@@ -369,7 +369,8 @@ AST_Scope.DEFMETHOD("references", function(sym){
 });
 
 AST_Symbol.DEFMETHOD("unmangleable", function(options){
-    return this.definition().unmangleable(options);
+    var def = this.definition();
+    return def && def.unmangleable(options);
 });
 
 // property accessors are not mangleable