diff --git a/AltairZ80/s100_2sio.c b/AltairZ80/s100_2sio.c
index 6c4ca125..89cacc1d 100644
--- a/AltairZ80/s100_2sio.c
+++ b/AltairZ80/s100_2sio.c
@@ -546,6 +546,10 @@ static t_stat m2sio_detach(UNIT *uptr)
 {
     M2SIO_CTX *xptr;
 
+    if (uptr->dptr == NULL) {
+        return SCPE_IERR;
+    }
+
     sim_debug(VERBOSE_MSG, uptr->dptr, "detach.\n");
 
     if (uptr->flags & UNIT_ATT) {