diff --git a/sim_tmxr.c b/sim_tmxr.c
index 4397b76e..6efff478 100644
--- a/sim_tmxr.c
+++ b/sim_tmxr.c
@@ -4456,7 +4456,8 @@ if (lp->txlog) {                                        /* logging? */
     lp->txlog = NULL;
     lp->txlogname = NULL;
     }
-lp->mp->uptr->filename = tmxr_mux_attach_string (lp->mp->uptr->filename, lp->mp);
+if (mp->uptr)
+    lp->mp->uptr->filename = tmxr_mux_attach_string (lp->mp->uptr->filename, lp->mp);
 return SCPE_OK;
 }