--- xfce4-session-4.12.1/xfce4-session/xfsm-systemd.c	2014-08-28 18:52:46.000000000 +0300
+++ xfsm-systemd.c	2018-09-15 14:12:45.231665058 +0300
@@ -110,13 +110,16 @@
 xfsm_systemd_lock_screen (GError **error)
 {
   XfconfChannel *channel;
-  gboolean       ret = TRUE;
 
   channel = xfsm_open_config ();
   if (xfconf_channel_get_bool (channel, "/shutdown/LockScreen", FALSE))
-      ret = g_spawn_command_line_async ("xflock4", error);
-
-  return ret;
+    {
+      gboolean ret;
+      gint exit_status;
+      ret = g_spawn_command_line_sync ("xflock4", NULL, NULL, &exit_status, error);
+      if (ret == FALSE || exit_status != 0) return FALSE;
+    }
+  return TRUE;
 }