diff --git a/drivers/isdn/hardware/mISDN/mISDNipac.c b/drivers/isdn/hardware/mISDN/mISDNipac.c
index 4d78f870435e..71e635d6c64a 100644
--- a/drivers/isdn/hardware/mISDN/mISDNipac.c
+++ b/drivers/isdn/hardware/mISDN/mISDNipac.c
@@ -710,7 +710,7 @@ isac_release(struct isac_hw *isac)
 {
 	if (isac->type & IPAC_TYPE_ISACX)
 		WriteISAC(isac, ISACX_MASK, 0xff);
-	else
+	else if (isac->type != 0)
 		WriteISAC(isac, ISAC_MASK, 0xff);
 	if (isac->dch.timer.function != NULL) {
 		del_timer(&isac->dch.timer);