diff --git a/src/gsm/auth_comp128v1.c b/src/gsm/auth_comp128v1.c
index a314dc2..ded3f8a 100644
--- a/src/gsm/auth_comp128v1.c
+++ b/src/gsm/auth_comp128v1.c
@@ -30,6 +30,7 @@
 			  struct osmo_sub_auth_data2 *aud,
 			  const uint8_t *_rand)
 {
+	OSMO_ASSERT(aud->algo == OSMO_AUTH_ALG_COMP128v1);
 	comp128v1(aud->u.gsm.ki, _rand, vec->sres, vec->kc);
 	vec->auth_types = OSMO_AUTH_TYPE_GSM;