diff --git a/CHANGELOG.md b/CHANGELOG.md
index 191caba699..ceb4d32fc8 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -11,6 +11,11 @@
 
 _Note: Gaps between patch versions are faulty/broken releases._
 
+## 3.0.5
+
+ * **Internal**
+  * More reliable default parameter scope.
+
 ## 3.0.4
 
  * **Bug Fix**
diff --git a/lib/6to5/transformation/transformers/es6/parameters.default.js b/lib/6to5/transformation/transformers/es6/parameters.default.js
index 9b08d6bed8..7d5bf97e92 100644
--- a/lib/6to5/transformation/transformers/es6/parameters.default.js
+++ b/lib/6to5/transformation/transformers/es6/parameters.default.js
@@ -1,7 +1,8 @@
 "use strict";
 
-var util = require("../../../util");
-var t    = require("../../../types");
+var traverse = require("../../../traverse");
+var util     = require("../../../util");
+var t        = require("../../../types");
 
 var hasDefaults = function (node) {
   for (var i = 0; i < node.params.length; i++) {
@@ -10,6 +11,15 @@ var hasDefaults = function (node) {
   return false;
 };
 
+var iifeVisitor = {
+  enter: function (node, parent, scope, context, state) {
+    if (t.isReferencedIdentifier(node, parent) && scope.hasOwn(node.name)) {
+      state.iife = true;
+      context.stop();
+    }
+  }
+};
+
 exports.Function = function (node, parent, scope) {
   if (!hasDefaults(node)) return;
 
@@ -23,6 +33,8 @@ exports.Function = function (node, parent, scope) {
 
   var lastNonDefaultParam = 0;
 
+  var state = { iife: false, scope: scope };
+
   for (var i = 0; i < node.params.length; i++) {
     var param = node.params[i];
 
@@ -36,10 +48,12 @@ exports.Function = function (node, parent, scope) {
 
     node.params[i] = scope.generateUidIdentifier("x");
 
-    // we're accessing a variable that's already defined within this function
-    var localDeclar = scope.get(left.name, true);
-    if (localDeclar !== left) {
-      iife = true;
+    if (!state.iife) {
+      if (t.isIdentifier(right) && scope.hasOwn(right.name)) {
+        state.iife = true;
+      } else {
+        traverse(right, iifeVisitor, scope, state);
+      }
     }
 
     var defNode = util.template("default-parameter", {
@@ -55,7 +69,7 @@ exports.Function = function (node, parent, scope) {
   // we need to cut off all trailing default parameters
   node.params = node.params.slice(0, lastNonDefaultParam);
 
-  if (iife) {
+  if (state.iife) {
     var container = t.functionExpression(null, [], node.body, node.generator);
     container._aliasFunction = true;