diff --git a/bitcoin/tx.c b/bitcoin/tx.c
index 27d0ed2f5..bface0c66 100644
--- a/bitcoin/tx.c
+++ b/bitcoin/tx.c
@@ -13,7 +13,7 @@
 
 #define SEGREGATED_WITNESS_FLAG 0x1
 
-int bitcoin_tx_add_output(struct bitcoin_tx *tx, u8 *script,
+int bitcoin_tx_add_output(struct bitcoin_tx *tx, const u8 *script,
 			  struct amount_sat *amount)
 {
 	size_t i = tx->wtx->num_outputs;
diff --git a/bitcoin/tx.h b/bitcoin/tx.h
index a4c2e53df..785fdde65 100644
--- a/bitcoin/tx.h
+++ b/bitcoin/tx.h
@@ -71,7 +71,7 @@ bool bitcoin_txid_to_hex(const struct bitcoin_txid *txid,
 struct bitcoin_tx *pull_bitcoin_tx(const tal_t *ctx,
 				   const u8 **cursor, size_t *max);
 
-int bitcoin_tx_add_output(struct bitcoin_tx *tx, u8 *script,
+int bitcoin_tx_add_output(struct bitcoin_tx *tx, const u8 *script,
 			  struct amount_sat *amount);
 
 int bitcoin_tx_add_input(struct bitcoin_tx *tx, const struct bitcoin_txid *txid,