diff --git a/lodash.js b/lodash.js
index 26a539acd..5fa267294 100644
--- a/lodash.js
+++ b/lodash.js
@@ -1455,7 +1455,7 @@
      * @returns {boolean} Returns `true` if the entry was removed successfully, else `false`.
      */
     function mapDelete(key) {
-      return delete this.__data__[key];
+      return this.has(key) && delete this.__data__[key];
     }
 
     /**
@@ -1468,7 +1468,7 @@
      * @returns {*} Returns the cached value.
      */
     function mapGet(key) {
-      return this.__data__[key];
+      return key == '__proto__' ? undefined : this.__data__[key];
     }
 
     /**
diff --git a/test/test.js b/test/test.js
index b66e42b20..d0747505e 100644
--- a/test/test.js
+++ b/test/test.js
@@ -7959,20 +7959,31 @@
       deepEqual(actual, shadowProps);
     });
 
-    test('should expose a `cache` object on the `memoized` function which implements `Map` interface', 2, function() {
+    test('should expose a `cache` object on the `memoized` function which implements `Map` interface', 18, function() {
       _.times(2, function(index) {
         var resolver = index ? _.identity : null,
-            memoized = _.memoize(_.identity, resolver);
+            memoized = _.memoize(function(value) { return 'value:' + value; }, resolver),
+            cache = memoized.cache;
 
         memoized('a');
 
-        deepEqual(_.functions(memoized.cache).sort(), ['delete', 'get', 'has', 'set']);
+        strictEqual(cache.has('a'), true);
+        strictEqual(cache.get('a'), 'value:a');
+        strictEqual(cache['delete']('a'), true);
+        strictEqual(cache['delete']('b'), false);
+
+        strictEqual(cache.set('b', 'value:b'), cache);
+        strictEqual(cache.has('b'), true);
+        strictEqual(cache.get('b'), 'value:b');
+        strictEqual(cache['delete']('b'), true);
+        strictEqual(cache['delete']('a'), false);
       });
     });
 
-    test('should skip the `__proto__` key', 4, function() {
+    test('should skip the `__proto__` key', 8, function() {
       _.times(2, function(index) {
         var count = 0,
+            key = '__proto__',
             resolver = index && _.identity;
 
         var memoized = _.memoize(function() {
@@ -7980,11 +7991,15 @@
           return [];
         }, resolver);
 
-        memoized('__proto__');
-        memoized('__proto__');
+        var cache = memoized.cache;
+
+        memoized(key);
+        memoized(key);
 
         strictEqual(count, 2);
-        ok(!(memoized.cache instanceof Array));
+        strictEqual(cache.get(key), undefined);
+        strictEqual(cache['delete'](key), false);
+        ok(!(cache.__data__ instanceof Array));
       });
     });