Index: wp-includes/cache.php
===================================================================
--- wp-includes/cache.php	(revision 21293)
+++ wp-includes/cache.php	(working copy)
@@ -323,6 +323,9 @@
 	 * @return false|int False on failure, the item's new value on success.
 	 */
 	function decr( $key, $offset = 1, $group = 'default' ) {
+		if ( empty( $group ) )
+			$group = 'default';
+		
 		if ( ! $this->_exists( $key, $group ) )
 			return false;