Index: wp-includes/query.php
===================================================================
--- wp-includes/query.php	(revision 32489)
+++ wp-includes/query.php	(working copy)
@@ -1867,6 +1867,10 @@
 				}
 
 				$term = $q[$t->query_var];
+                
+				if ( is_array($term) ) {
+					$term = implode(',',$term);
+				}
 
 				if ( strpos($term, '+') !== false ) {
 					$terms = preg_split( '/[+]+/', $term );