diff --git a/lib/interactive-auth.js b/lib/interactive-auth.js
index 7392753f0..8ba89516b 100644
--- a/lib/interactive-auth.js
+++ b/lib/interactive-auth.js
@@ -18,6 +18,8 @@ limitations under the License.
 /** @module interactive-auth */
 var q = require("q");
 
+var utils = require("./utils");
+
 /**
  * Abstracts the logic used to drive the interactive auth process.
  *
@@ -116,7 +118,7 @@ InteractiveAuth.prototype = {
         var auth = {
             session: this._data.session,
         };
-        Object.assign(auth, authData);
+        utils.extend(auth, authData);
 
         this._doRequest(auth);
     },
diff --git a/lib/utils.js b/lib/utils.js
index 942503d20..523cde013 100644
--- a/lib/utils.js
+++ b/lib/utils.js
@@ -337,6 +337,9 @@ var deepCompare = module.exports.deepCompare = function(x, y) {
  *
  * All enumerable properties, included inherited ones, are copied.
  *
+ * This is approximately equivalent to ES6's Object.assign, except
+ * that the latter doesn't copy inherited properties.
+ *
  * @param {Object} target  The object that will receive new properties
  * @param {...Object} source  Objects from which to copy properties
  *