diff --git a/openbsc/src/gsm_04_11.c b/openbsc/src/gsm_04_11.c
index 4f19e2c..d4572ca 100644
--- a/openbsc/src/gsm_04_11.c
+++ b/openbsc/src/gsm_04_11.c
@@ -1018,7 +1018,7 @@
 	struct gsm_trans *trans;
 	u_int8_t *data, *rp_ud_len;
 	u_int8_t msg_ref = 42;
-	u_int8_t transaction_id;
+	int transaction_id;
 	int rc;
 
 	transaction_id = trans_assign_trans_id(lchan->subscr, GSM48_PDISC_SMS, 0);