diff --git misc/ajax.js misc/ajax.js
index 90c3bb8bbef3e306cfa2b176920a9eff5adf18b0..1dd46969f957104b4d23526e976d2f63a90de62d 100644
--- misc/ajax.js
+++ misc/ajax.js
@@ -347,7 +347,14 @@ 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) {
+    if (v !== null && typeof v == "object") {
+      var name = this.element.name;
+      $.each(v, function(i, value) {
+        var key = name.replace(/\[\]/, '[' + value + ']');
+        options.extraData[key] = value;
+      });
+    }
+    else if (v !== null) {
       options.extraData[this.element.name] = v;
     }
   }