diff --git a/lightningd/channel_control.c b/lightningd/channel_control.c
index 9735e72a6699..55b8a381c266 100644
--- a/lightningd/channel_control.c
+++ b/lightningd/channel_control.c
@@ -621,7 +621,7 @@ static void send_splice_tx(struct channel *channel,
 	u8* tx_bytes = linearize_tx(tmpctx, tx);
 
 	log_debug(channel->log,
-		  "Broadcasting splice tx %s for channel %s. Final weight %lu",
+		  "Broadcasting splice tx %s for channel %s. Final weight %zu",
 		  tal_hex(tmpctx, tx_bytes),
 		  fmt_channel_id(tmpctx, &channel->cid),
 		  bitcoin_tx_weight(tx));