diff --git a/drivers/isdn/capi/kcapi.c b/drivers/isdn/capi/kcapi.c
index a4ceb61c5b6035fef835759d0761c733dea3fde8..ed9ee2bbf232e978e9ef7e67f5113ddbf66bbd29 100644
--- a/drivers/isdn/capi/kcapi.c
+++ b/drivers/isdn/capi/kcapi.c
@@ -565,6 +565,11 @@ 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;