diff --git a/lib/6to5/transformation/transformers/constants.js b/lib/6to5/transformation/transformers/constants.js
index 7c342edee7..965d33dda3 100644
--- a/lib/6to5/transformation/transformers/constants.js
+++ b/lib/6to5/transformation/transformers/constants.js
@@ -7,12 +7,12 @@ exports.BlockStatement =
 exports.ForInStatement =
 exports.ForOfStatement =
 exports.ForStatement = function (node, parent, file) {
-  var constants = [];
+  var constants = {};
 
   var check = function (node, names, parent) {
     _.each(names, function (name) {
       if (!_.has(constants, name)) return;
-      if (t.isBlockStatement(parent) && parent !== constants[name]) return;
+      if (parent && t.isBlockStatement(parent) && parent !== constants[name]) return;
 
       throw file.errorWithNode(node, name + " is read-only");
     });
@@ -38,13 +38,13 @@ exports.ForStatement = function (node, parent, file) {
     }
   });
 
-  if (!constants.length) return;
+  if (_.isEmpty(constants)) return;
 
-  traverse(node, function (child) {
+  traverse(node, function (child, parent) {
     if (child._ignoreConstant) return;
 
-    if (t.isVariableDeclarator(child) || t.isDeclaration(child) || t.isAssignmentExpression(child)) {
-      check(child, getIds(child));
+    if (t.isDeclaration(child) || t.isAssignmentExpression(child)) {
+      check(child, getIds(child), parent);
     }
   });
 };