diff --git a/drivers/isdn/hardware/mISDN/hfcsusb.c b/drivers/isdn/hardware/mISDN/hfcsusb.c
index b3b7e2879bac..8700474747e8 100644
--- a/drivers/isdn/hardware/mISDN/hfcsusb.c
+++ b/drivers/isdn/hardware/mISDN/hfcsusb.c
@@ -97,8 +97,10 @@ static int write_reg(struct hfcsusb *hw, __u8 reg, __u8 val)
 			hw->name, __func__, reg, val);
 
 	spin_lock(&hw->ctrl_lock);
-	if (hw->ctrl_cnt >= HFC_CTRL_BUFSIZE)
+	if (hw->ctrl_cnt >= HFC_CTRL_BUFSIZE) {
+		spin_unlock(&hw->ctrl_lock);
 		return 1;
+	}
 	buf = &hw->ctrl_buff[hw->ctrl_in_idx];
 	buf->hfcs_reg = reg;
 	buf->reg_val = val;