diff --git a/common/bolt12.c b/common/bolt12.c
index e31b97f8c..4f1a5ab33 100644
--- a/common/bolt12.c
+++ b/common/bolt12.c
@@ -432,7 +432,7 @@ bool bolt12_has_prefix(const char *str)
 }
 
 /* Inclusive span of tlv range >= minfield and <= maxfield */
-size_t tlv_span(const u8 *tlvstream, size_t minfield, size_t maxfield,
+size_t tlv_span(const u8 *tlvstream, u64 minfield, u64 maxfield,
 		size_t *startp)
 {
 	const u8 *cursor = tlvstream;
diff --git a/common/bolt12.h b/common/bolt12.h
index 80b43bb57..277a3f80b 100644
--- a/common/bolt12.h
+++ b/common/bolt12.h
@@ -135,7 +135,7 @@ bool bolt12_has_prefix(const char *str);
  *
  * Returns length, so 0 means nothing found.
 */
-size_t tlv_span(const u8 *tlvstream, size_t minfield, size_t maxfield,
+size_t tlv_span(const u8 *tlvstream, u64 minfield, u64 maxfield,
 		size_t *start);
 
 /* Get offer_id referred to by various structures. */