diff -Naur simh-3.8.1/ALTAIR/altair_dsk.c simh-3.8.1-mod/ALTAIR/altair_dsk.c
--- simh-3.8.1/ALTAIR/altair_dsk.c	2005-08-26 21:47:22.000000000 +0300
+++ simh-3.8.1-mod/ALTAIR/altair_dsk.c	2009-11-14 21:22:23.000000000 +0200
@@ -325,6 +325,8 @@
                         cur_sect[cur_disk]);*/
         pos = DSK_TRACSIZE * cur_track[cur_disk];
         pos += DSK_SECTSIZE * cur_sect[cur_disk];
+	if ((uptr == NULL) || (uptr->fileref == NULL))
+		return 0;
         rtn = fseek(uptr -> fileref, pos, 0);
         rtn = fread(dskbuf, 137, 1, uptr -> fileref);
         cur_byte[cur_disk] = 1;