diff --git a/index.js b/index.js
index 489b99c..b1b5295 100644
--- a/index.js
+++ b/index.js
@@ -16,7 +16,7 @@ FakeMap.prototype = {
     return key[this._key];
   },
   set: function setMap(key, value) {
-    if (!Object.isFrozen(key)) {
+    if (Object.isExtensible(key)) {
       Object.defineProperty(key, this._key, {
         value: value,
         configurable: true,
diff --git a/test/index.js b/test/index.js
index a0cd1a7..5714d0b 100644
--- a/test/index.js
+++ b/test/index.js
@@ -332,6 +332,20 @@ describe('Generic', function () {
         'eql({ foo: 1, bar: -> }, { foo: 1, bar: <- }) === true');
     });
 
+    it('returns true with non-extensible objects', function () {
+      var objectA = Object.preventExtensions({ foo: 1 });
+      var objectB = Object.preventExtensions({ foo: 1 });
+      assert(eql(objectA, objectB) === true,
+        'eql(Object.preventExtensions({ foo: 1 }), Object.preventExtensions({ foo: 1 })) === true');
+    });
+
+    it('returns true with sealed objects', function () {
+      var objectA = Object.seal({ foo: 1 });
+      var objectB = Object.seal({ foo: 1 });
+      assert(eql(objectA, objectB) === true,
+        'eql(Object.seal({ foo: 1 }), Object.seal({ foo: 1 })) === true');
+    });
+
     it('returns true with frozen objects', function () {
       var objectA = Object.freeze({ foo: 1 });
       var objectB = Object.freeze({ foo: 1 });
diff --git a/test/new-ecmascript-types.js b/test/new-ecmascript-types.js
index 7c28b95..2195dbe 100644
--- a/test/new-ecmascript-types.js
+++ b/test/new-ecmascript-types.js
@@ -338,14 +338,23 @@ describe('ES2015 Specific', function () {
       assert(eql(promiseResolve, promiseResolve), 'eql(promiseResolve, promiseResolve)');
       assert(eql(promiseReject, promiseReject), 'eql(promiseReject, promiseReject)');
       assert(eql(promisePending, promisePending), 'eql(promisePending, promisePending)');
+
+      promiseReject.catch(function () {});
     });
 
 
     it('returns false for different promises', function () {
       assert(eql(Promise.resolve(), Promise.resolve()) === false,
         'eql(Promise.resolve(), Promise.resolve()) === false');
-      assert(eql(Promise.reject(), Promise.reject()) === false,
+
+      var promiseRejectA = Promise.reject();
+      var promiseRejectB = Promise.reject();
+      assert(eql(promiseRejectA, promiseRejectB) === false,
         'eql(Promise.reject(), Promise.reject()) === false');
+
+      promiseRejectA.catch(function () {});
+      promiseRejectB.catch(function () {});
+
       assert(eql(new Promise(emptyFunction), new Promise(emptyFunction)) === false,
         'eql(new Promise(emptyFunction), new Promise(emptyFunction)) === false');
     });