Index: wp-admin/includes/class-wp-filesystem-direct.php
===================================================================
--- wp-admin/includes/class-wp-filesystem-direct.php	(revision 11745)
+++ wp-admin/includes/class-wp-filesystem-direct.php	(working copy)
@@ -206,6 +206,7 @@
 	}
 
 	function mkdir($path, $chmod = false, $chown = false, $chgrp = false){
+		$path = untrailingslashit($path);
 		if ( ! @mkdir($path) )
 			return false;
 		$this->chmod($path, $chmod);