diff --git a/lib/utils/map.js b/lib/utils/map.js
index fb491989..f6070383 100644
--- a/lib/utils/map.js
+++ b/lib/utils/map.js
@@ -17,7 +17,7 @@ function Map() {
   if (!(this instanceof Map))
     return new Map();
 
-  this.map = {};
+  this.map = Object.create(null);
   this.size = 0;
 }
 
@@ -55,6 +55,8 @@ Map.prototype.values = function values() {
  */
 
 Map.prototype.get = function get(key) {
+  if (key === '__proto__')
+    return;
   return this.map[key];
 };
 
@@ -65,6 +67,8 @@ Map.prototype.get = function get(key) {
  */
 
 Map.prototype.has = function has(key) {
+  if (key === '__proto__')
+    return false;
   return this.map[key] !== undefined;
 };
 
@@ -80,6 +84,9 @@ Map.prototype.set = function set(key, value) {
 
   assert(value !== undefined);
 
+  if (key === '__proto__')
+    return;
+
   this.map[key] = value;
 
   if (item === undefined) {
@@ -113,10 +120,21 @@ Map.prototype.remove = function remove(key) {
  */
 
 Map.prototype.reset = function reset() {
-  this.map = {};
+  this.map = Object.create(null);
   this.size = 0;
 };
 
+/**
+ * Insert a key.
+ * Equivalent to `this.set([key], true)`.
+ * @param {String} key
+ * @returns {Boolean}
+ */
+
+Map.prototype.insert = function insert(key) {
+  return this.set(key, true);
+};
+
 /*
  * Expose
  */