Index: xmlrpc.php
===================================================================
--- xmlrpc.php	(revision 4794)
+++ xmlrpc.php	(working copy)
@@ -1432,7 +1432,7 @@
 		foreach($catids as $catid) {
 			$categories[] = array(
 				'categoryName' => get_cat_name($catid),
-				'categoryId' => $catid,
+				'categoryId' => (string) $catid,
 				'isPrimary' => $isPrimary
 			);
 			$isPrimary = false;