Index: wp-includes/capabilities.php
===================================================================
--- wp-includes/capabilities.php	(revision 19722)
+++ wp-includes/capabilities.php	(working copy)
@@ -500,6 +500,9 @@
 		if ( !$value )
 			return false;
 
+		if ( $value === 0 )
+			return NULL;
+
 		switch ( $field ) {
 			case 'id':
 				$user_id = $value;