Index: wp-includes/class-wp-xmlrpc-server.php
===================================================================
--- wp-includes/class-wp-xmlrpc-server.php	(revision 37961)
+++ wp-includes/class-wp-xmlrpc-server.php	(working copy)
@@ -3053,7 +3053,7 @@
 
 		// Make sure the user is allowed to add a category.
 		if ( !current_user_can('manage_categories') )
-			return new IXR_Error(401, __('Sorry, you are not allowed to add a category.'));
+			return new IXR_Error(401, __('Sorry, you are not allowed to add categories.'));
 
 		// If no slug was provided make it empty so that
 		// WordPress will generate one.