diff --git a/openbsc/include/openbsc/gsm_data.h b/openbsc/include/openbsc/gsm_data.h
index a50d9f21d..a6bc16a14 100644
--- a/openbsc/include/openbsc/gsm_data.h
+++ b/openbsc/include/openbsc/gsm_data.h
@@ -608,7 +608,7 @@ struct gsm_auth_tuple {
 	int use_count;
 	int key_seq;
 	u_int8_t rand[16];
-	u_int8_t sres[8];
+	u_int8_t sres[4];
 	u_int8_t kc[8];
 };