diff --git a/test/functions.js b/test/functions.js
index 3c9f6a2c1..230ebea9a 100644
--- a/test/functions.js
+++ b/test/functions.js
@@ -52,6 +52,13 @@ $(document).ready(function() {
     var fastFib = _.memoize(fib);
     equals(fib(10), 55, 'a memoized version of fibonacci produces identical results');
     equals(fastFib(10), 55, 'a memoized version of fibonacci produces identical results');
+
+    var o = function(str) {
+      return str;
+    };
+    var fastO = _.memoize(o);
+    equals(o('toString'), 'toString', 'checks hasOwnProperty');
+    equals(fastO('toString'), 'toString', 'checks hasOwnProperty');
   });
 
   asyncTest("functions: delay", 2, function() {
diff --git a/underscore.js b/underscore.js
index 4a9400d7c..343f335be 100644
--- a/underscore.js
+++ b/underscore.js
@@ -426,7 +426,7 @@
     hasher = hasher || _.identity;
     return function() {
       var key = hasher.apply(this, arguments);
-      return key in memo ? memo[key] : (memo[key] = func.apply(this, arguments));
+      return hasOwnProperty.call(memo, key) ? memo[key] : (memo[key] = func.apply(this, arguments));
     };
   };