diff --git a/drivers/isdn/hardware/avm/c4.c b/drivers/isdn/hardware/avm/c4.c
index 8710cf6214d9fc9487841547a4139625948d51c1..4bbbbe688077558f1793c746e0f95380d8b65f17 100644
--- a/drivers/isdn/hardware/avm/c4.c
+++ b/drivers/isdn/hardware/avm/c4.c
@@ -678,7 +678,9 @@ static irqreturn_t c4_handle_interrupt(avmcard *card)
                 for (i=0; i < card->nr_controllers; i++) {
 			avmctrl_info *cinfo = &card->ctrlinfo[i];
 			memset(cinfo->version, 0, sizeof(cinfo->version));
+			spin_lock_irqsave(&card->lock, flags);
 			capilib_release(&cinfo->ncci_head);
+			spin_unlock_irqrestore(&card->lock, flags);
 			capi_ctr_reseted(&cinfo->capi_ctrl);
 		}
 		card->nlogcontr = 0;