Index: wp-admin/includes/post.php
===================================================================
--- wp-admin/includes/post.php	(revision 40060)
+++ wp-admin/includes/post.php	(working copy)
@@ -1462,6 +1462,9 @@
 	$time = $lock[0];
 	$user = isset( $lock[1] ) ? $lock[1] : get_post_meta( $post->ID, '_edit_last', true );
 
+	if ( get_userdata( $user ) === false )
+		return false;
+
 	/** This filter is documented in wp-admin/includes/ajax-actions.php */
 	$time_window = apply_filters( 'wp_check_post_lock_window', 150 );