diff --git a/common/psbt_open.c b/common/psbt_open.c
index 71a691048..55b51e79f 100644
--- a/common/psbt_open.c
+++ b/common/psbt_open.c
@@ -351,7 +351,7 @@ void psbt_input_add_max_witness_len(struct wally_psbt_input *input,
 }
 
 
-bool psbt_input_get_max_witness_len(struct wally_psbt_input *input,
+bool psbt_input_get_max_witness_len(const struct wally_psbt_input *input,
 				    u16 *max_witness_len)
 {
 	size_t value_len;
diff --git a/common/psbt_open.h b/common/psbt_open.h
index 776811e13..8c9452881 100644
--- a/common/psbt_open.h
+++ b/common/psbt_open.h
@@ -127,7 +127,7 @@ void psbt_input_add_max_witness_len(struct wally_psbt_input *input,
  *
  * Returns false if key not present */
 WARN_UNUSED_RESULT bool
-psbt_input_get_max_witness_len(struct wally_psbt_input *input,
+psbt_input_get_max_witness_len(const struct wally_psbt_input *input,
 			       u16 *max_witness_len);
 
 /* psbt_has_required_fields - Validates psbt field completion