Index: wp-includes/class-IXR.php
===================================================================
--- wp-includes/class-IXR.php	(revision 15615)
+++ wp-includes/class-IXR.php	(working copy)
@@ -441,7 +441,7 @@
         } else {
             // It's a function - does it exist?
             if (is_array($method)) {
-                if (!method_exists($method[0], $method[1])) {
+                if (!is_callable(array($method[0], $method[1]))) {
                     return new IXR_Error(-32601, 'server error. requested object method "'.$method[1].'" does not exist.');
                 }
             } else if (!function_exists($method)) {