diff --git a/drivers/isdn/capi/kcapi.c b/drivers/isdn/capi/kcapi.c
index cb0afe89716236..7313454e403a63 100644
--- a/drivers/isdn/capi/kcapi.c
+++ b/drivers/isdn/capi/kcapi.c
@@ -480,6 +480,11 @@ int detach_capi_ctr(struct capi_ctr *ctr)
 
 	ctr_down(ctr, CAPI_CTR_DETACHED);
 
+	if (ctr->cnr < 1 || ctr->cnr - 1 >= CAPI_MAXCONTR) {
+		err = -EINVAL;
+		goto unlock_out;
+	}
+
 	if (capi_controller[ctr->cnr - 1] != ctr) {
 		err = -EINVAL;
 		goto unlock_out;