Index: wp-includes/query.php
===================================================================
--- wp-includes/query.php	(revision 22098)
+++ wp-includes/query.php	(working copy)
@@ -1463,7 +1463,8 @@
 		if ( '' != $qv['subpost_id'] )
 			$qv['attachment_id'] = $qv['subpost_id'];
 
-		$qv['attachment_id'] = absint($qv['attachment_id']);
+		if ( is_numeric($qv['attachment_id']) )
+			$qv['attachment_id'] = absint($qv['attachment_id']);
 
 		if ( ('' != $qv['attachment']) || !empty($qv['attachment_id']) ) {
 			$this->is_single = true;