diff --git a/bitcoin/tx.c b/bitcoin/tx.c
index 949f3a1de..dea1ca143 100644
--- a/bitcoin/tx.c
+++ b/bitcoin/tx.c
@@ -285,7 +285,7 @@ struct amount_asset bitcoin_tx_output_get_amount(const struct bitcoin_tx *tx,
 	return wally_tx_output_get_amount(&tx->wtx->outputs[outnum]);
 }
 
-void bitcoin_tx_output_get_amount_sat(struct bitcoin_tx *tx, int outnum,
+void bitcoin_tx_output_get_amount_sat(const struct bitcoin_tx *tx, int outnum,
 				      struct amount_sat *amount)
 {
 	struct amount_asset asset_amt;
diff --git a/bitcoin/tx.h b/bitcoin/tx.h
index 3a56dc0e6..2db9e9506 100644
--- a/bitcoin/tx.h
+++ b/bitcoin/tx.h
@@ -156,7 +156,7 @@ u8 *bitcoin_tx_output_get_witscript(const tal_t *ctx, const struct bitcoin_tx *t
  * Internally we use a `wally_tx` to represent the transaction. The
  * satoshi amount isn't a struct amount_sat, so we need a conversion
  */
-void bitcoin_tx_output_get_amount_sat(struct bitcoin_tx *tx, int outnum,
+void bitcoin_tx_output_get_amount_sat(const struct bitcoin_tx *tx, int outnum,
 				      struct amount_sat *amount);
 /**
  * Helper to just get an amount_sat for the output amount.