diff --git a/lib/6to5/traverse/scope.js b/lib/6to5/traverse/scope.js
index 1747f29d44..cb45f4d2c6 100644
--- a/lib/6to5/traverse/scope.js
+++ b/lib/6to5/traverse/scope.js
@@ -112,7 +112,7 @@ Scope.prototype.push = function (name, id, init) {
       init: init
     };
   } else {
-    throw new Error("wtf");
+    throw new TypeError("cannot add a declaration here in node type " + block.type);
   }
 };