Index: wp-includes/capabilities.php
===================================================================
--- wp-includes/capabilities.php	(revision 23382)
+++ wp-includes/capabilities.php	(working copy)
@@ -993,7 +993,17 @@
 function map_meta_cap( $cap, $user_id ) {
 	$args = array_slice( func_get_args(), 2 );
 	$caps = array();
-
+	
+	if ( ! isset( $args[0] ) ) {
+		global $post;
+		if ( is_object( $post ) ) {
+			$args[0] = $post->ID;
+		}
+		else {
+			$args[0] = $_POST['post_ID'];
+		}
+	}
+	
 	switch ( $cap ) {
 	case 'remove_user':
 		$caps[] = 'remove_users';