Index: wp-includes/pluggable.php
===================================================================
--- wp-includes/pluggable.php	(revision 23178)
+++ wp-includes/pluggable.php	(working copy)
@@ -650,6 +650,10 @@
  * @param bool $remember Whether to remember the user
  */
 function wp_set_auth_cookie($user_id, $remember = false, $secure = '') {
+
+	if( ! is_int( $user_id ) || $user_id < 1 )
+		return;
+
 	if ( $remember ) {
 		$expiration = $expire = time() + apply_filters('auth_cookie_expiration', 1209600, $user_id, $remember);
 	} else {