Index: wp-includes/query.php
===================================================================
--- wp-includes/query.php	(revision 18569)
+++ wp-includes/query.php	(working copy)
@@ -1467,7 +1467,7 @@
 		} else {
 		// Look for archive queries.  Dates, categories, authors, search, post type archives.
 
-			if ( !empty($qv['s']) ) {
+			if ( isset($qv['s']) ) {
 				$this->is_search = true;
 			}