diff --git a/drivers/isdn/capi/kcapi.c b/drivers/isdn/capi/kcapi.c
index 18de41a266eb..6175ff7ec749 100644
--- a/drivers/isdn/capi/kcapi.c
+++ b/drivers/isdn/capi/kcapi.c
@@ -563,6 +563,9 @@ int detach_capi_ctr(struct capi_ctr *ctr)
 
 	mutex_lock(&capi_controller_lock);
 
+	if (ncontrollers == 0)
+		goto unlock_out;
+
 	ctr_down(ctr, CAPI_CTR_DETACHED);
 
 	if (capi_controller[ctr->cnr - 1] != ctr) {