diff --git a/sim_tmxr.c b/sim_tmxr.c
index d0c6b717..e7474e73 100644
--- a/sim_tmxr.c
+++ b/sim_tmxr.c
@@ -3437,7 +3437,8 @@ if (lp->txlog == NULL) {                                /* error? */
     free (lp->txlogname);                               /* free buffer */
     return SCPE_OPENERR;
     }
-lp->mp->uptr->filename = tmxr_mux_attach_string (lp->mp->uptr->filename, lp->mp);
+if (mp->uptr)                                           /* attached?, then update attach string */
+    lp->mp->uptr->filename = tmxr_mux_attach_string (lp->mp->uptr->filename, lp->mp);
 return SCPE_OK;
 }