diff --git a/lib/quicly.c b/lib/quicly.c
index 82ba627..8c0163e 100644
--- a/lib/quicly.c
+++ b/lib/quicly.c
@@ -4713,7 +4713,7 @@ static int handle_ack_frame(quicly_conn_t *conn, struct st_quicly_handle_payload
             /* process newly acked packet */
             if (state->epoch != sent->ack_epoch)
                 return QUICLY_TRANSPORT_ERROR_PROTOCOL_VIOLATION;
-            int is_late_ack = 0;
+            int __attribute__((unused)) is_late_ack = 0;
             if (sent->ack_eliciting) {
                 includes_ack_eliciting = 1;
                 if (sent->cc_bytes_in_flight == 0) {