diff --git a/src/lib/base.html b/src/lib/base.html
index b12bfebe97..f58ec4b267 100644
--- a/src/lib/base.html
+++ b/src/lib/base.html
@@ -116,18 +116,19 @@
      * object to a target object.
      *
      * @method extend
-     * @param {Object} prototype Target object to copy properties to.
-     * @param {Object} api Source object to copy properties from.
-     * @return {Object} prototype object that was passed as first argument.
+     * @param {?Object} target Target object to copy properties to.
+     * @param {?Object} source Source object to copy properties from.
+     * @return {?Object} Target object that was passed as first argument or
+     *     source object if the target was null.
      */
-    extend: function(prototype, api) {
-      if (prototype && api) {
-        var n$ = Object.getOwnPropertyNames(api);
+    extend: function(target, source) {
+      if (target && source) {
+        var n$ = Object.getOwnPropertyNames(source);
         for (var i=0, n; (i<n$.length) && (n=n$[i]); i++) {
-          this.copyOwnProperty(n, api, prototype);
+          this.copyOwnProperty(n, source, target);
         }
       }
-      return prototype || api;
+      return target || source;
     },
 
     /**
@@ -138,9 +139,9 @@
      * to target and that accessor implementations are copied, use `extend`.
      *
      * @method mixin
-     * @param {Object} target Target object to copy properties to.
-     * @param {Object} source Source object to copy properties from.
-     * @return {Object} Target object that was passed as first argument.
+     * @param {!Object} target Target object to copy properties to.
+     * @param {?Object} source Source object to copy properties from.
+     * @return {!Object} Target object that was passed as first argument.
      */
     mixin: function(target, source) {
       for (var i in source) {