diff --git a/build.js b/build.js
index 92ca023a9..f065158c6 100755
--- a/build.js
+++ b/build.js
@@ -439,25 +439,19 @@
   if (isMobile) {
     // inline functions defined with `createIterator`
     lodash.functions(lodash).forEach(function(funcName) {
-      var reFunc = RegExp('( +var ' + funcName + ' *= *)((?:[a-zA-Z]+ *\\|\\| *)?)createIterator\\(((?:{|[a-zA-Z])[\\s\\S]+?)\\);\\n'),
-          parts = source.match(reFunc);
+      var reFunc = RegExp('(\\bvar ' + funcName.replace(/^_/, '') + ' *= *)createIterator\\(((?:{|[a-zA-Z])[\\s\\S]+?)\\);\\n');
 
       // skip if not defined with `createIterator`
-      if (!parts) {
+      if (!reFunc.test(source)) {
         return;
       }
-      // extract function's code
-      var code = funcName == 'keys'
-        ? '$1$2' + lodash._createIterator(Function('return ' + parts[3])())
-        : '$1' + lodash[funcName];
-
-      // format code
-      code = code.replace(/\n(?:.*)/g, function(match) {
+      // extract and format the function's code
+      var code = (lodash[funcName] + '').replace(/\n(?:.*)/g, function(match) {
         match = match.slice(1);
         return (match == '}' ? '\n  ' : '\n    ') + match;
-      }) + ';\n';
+      });
 
-      source = source.replace(reFunc, code);
+      source = source.replace(reFunc, '$1' + code + ';\n');
     });
 
     // remove `iteratorTemplate`
@@ -490,7 +484,7 @@
   }
 
   // remove pseudo private properties
-  source = source.replace(/(?:\s*\/\/.*)*\s*lodash\._(?:createIterator|iteratorTemplate)\b.+\n/g, '\n');
+  source = source.replace(/(?:\s*\/\/.*)*\s*lodash\._[^=]+=.+\n/g, '\n');
 
   /*--------------------------------------------------------------------------*/
 
diff --git a/lodash.js b/lodash.js
index 55c4c32f3..425d6e857 100644
--- a/lodash.js
+++ b/lodash.js
@@ -477,6 +477,21 @@
     // no operation performed
   }
 
+  /**
+   * A shim implementation of `Object.keys` that produces an array of the given
+   * object's enumerable own property names.
+   *
+   * @private
+   * @param {Object} object The object to inspect.
+   * @returns {Array} Returns a new array of property names.
+   */
+  var shimKeys = createIterator({
+    'args': 'object',
+    'exit': 'if (!objectTypes[typeof object] || object === null) throw TypeError()',
+    'init': '[]',
+    'inLoop': 'result.push(index)'
+  });
+
   /**
    * Used by `template()` to replace "escape" template delimiters with tokens.
    *
@@ -2643,12 +2658,12 @@
    * _.keys({ 'one': 1, 'two': 2, 'three': 3 });
    * // => ['one', 'two', 'three']
    */
-  var keys = nativeKeys || createIterator({
-    'args': 'object',
-    'exit': 'if (!objectTypes[typeof object] || object === null) throw TypeError()',
-    'init': '[]',
-    'inLoop': 'result.push(index)'
-  });
+  var keys = !nativeKeys ? shimKeys : function(object) {
+    // avoid iterating over the `prototype` property
+    return typeof object == 'function'
+      ? shimKeys(object)
+      : nativeKeys(object);
+  };
 
   /**
    * Creates an object composed of the specified properties. Property names may
@@ -3209,6 +3224,7 @@
   // add pseudo privates used and removed during the build process
   lodash._createIterator = createIterator;
   lodash._iteratorTemplate = iteratorTemplate;
+  lodash._shimKeys = shimKeys;
 
   /*--------------------------------------------------------------------------*/
 
diff --git a/test/test.js b/test/test.js
index e98c8ab09..8b4f8d417 100644
--- a/test/test.js
+++ b/test/test.js
@@ -306,6 +306,20 @@
       deepEqual(_.keys(shadowed).sort(),
         'constructor hasOwnProperty isPrototypeOf propertyIsEnumerable toLocaleString toString valueOf'.split(' '));
     });
+
+    test('skips the prototype property of functions (test in Firefox < 3.6, Opera > 9.50 - Opera < 11.60, and Safari < 5.1)', function() {
+      function Foo() {}
+      Foo.prototype.c = 3;
+
+      Foo.a = 1;
+      Foo.b = 2;
+
+      var expected = ['a', 'b'];
+      deepEqual(_.keys(Foo), expected);
+
+      Foo.prototype = { 'c': 3 };
+      deepEqual(_.keys(Foo), expected);
+    });
   }());
 
   /*--------------------------------------------------------------------------*/