diff --git a/drivers/isdn/mISDN/stack.c b/drivers/isdn/mISDN/stack.c
index 3e1532a180ff..0d05ec43012c 100644
--- a/drivers/isdn/mISDN/stack.c
+++ b/drivers/isdn/mISDN/stack.c
@@ -364,7 +364,7 @@ add_layer2(struct mISDNchannel *ch, struct mISDNstack *st)
 static int
 st_own_ctrl(struct mISDNchannel *ch, u_int cmd, void *arg)
 {
-	if (!ch->st || ch->st->layer1)
+	if (!ch->st || !ch->st->layer1)
 		return -EINVAL;
 	return ch->st->layer1->ctrl(ch->st->layer1, cmd, arg);
 }