diff --git a/src/scripts/require.js b/src/scripts/require.js
index 24c6a16c9..5656bfa82 100644
--- a/src/scripts/require.js
+++ b/src/scripts/require.js
@@ -67,7 +67,7 @@ var define;
     };
 
     define = function (id, factory) {
-        if (modules[id]) {
+        if (Object.prototype.hasOwnProperty.call(modules, id)) {
             throw 'module ' + id + ' already defined';
         }
 
diff --git a/test/test.require.js b/test/test.require.js
index b66e9abde..ace76169c 100644
--- a/test/test.require.js
+++ b/test/test.require.js
@@ -231,7 +231,7 @@ describe('require + define', function () {
         });
 
         // Adapted version of CommonJS test `hasOwnProperty`
-        xit('Test#018 : allows properties of Object.prototype as module names', () => {
+        it('Test#018 : allows properties of Object.prototype as module names', () => {
             expect(() => {
                 define('hasOwnProperty', jasmine.createSpy());
             }).not.toThrow();