diff --git a/misc/ajax.js b/misc/ajax.js
index c944ebbf24..c91e6860af 100644
--- a/misc/ajax.js
+++ b/misc/ajax.js
@@ -375,8 +375,17 @@ Drupal.ajax.prototype.beforeSend = function (xmlhttprequest, options) {
     // $.ajax() are already serialized prior to the element being disabled, so
     // this is only needed for IFRAME submissions.
     var v = $.fieldValue(this.element);
-    if (v !== null) {
-      options.extraData[this.element.name] = Drupal.checkPlain(v);
+    if (v !== null && typeof v === 'object') {
+      var name = this.element.name;
+      $.each(v, function (i, value) {
+        var key = name.replace(/\[\]/, '[' + value + ']');
+        options.extraData[key] = Drupal.checkPlain(value);
+      });
+    }
+    else {
+      if (v !== null) {
+        options.extraData[this.element.name] = Drupal.checkPlain(v);
+      }
     }
   }