diff --git a/src/rspro_util.c b/src/rspro_util.c
index 1d95aee..ab748f6 100644
--- a/src/rspro_util.c
+++ b/src/rspro_util.c
@@ -89,7 +89,7 @@ RsproPDU_t *rspro_dec_msg(struct msgb *msg)
 	//printf("decoding %s\n", msgb_hexdump(msg));
 	rval = ber_decode(NULL, &asn_DEF_RsproPDU, (void **) &pdu, msgb_l2(msg), msgb_l2len(msg));
 	if (rval.code != RC_OK) {
-		fprintf(stderr, "Failed to decode: %d. Consumed %lu of %u bytes\n",
+		fprintf(stderr, "Failed to decode: %d. Consumed %zu of %u bytes\n",
 			rval.code, rval.consumed, msgb_length(msg));
 		return NULL;
 	}