diff --git a/common/test/run-bolt12_merkle.c b/common/test/run-bolt12_merkle.c
index 228da66d08b1..fad71bbc70ff 100644
--- a/common/test/run-bolt12_merkle.c
+++ b/common/test/run-bolt12_merkle.c
@@ -326,7 +326,7 @@ int main(int argc, char *argv[])
 	invreq->invreq_metadata = tal_arrz(invreq, u8, 8);
 
 	/* Populate ->fields array, for merkle routine */
-	invreq->fields = tlv_make_fields(invreq, tlv_invoice_request);
+	tlv_update_fields(invreq, tlv_invoice_request, &invreq->fields);
 	merkle_tlv(invreq->fields, &test_m);
 
 	/* BOLT-offers #12:
diff --git a/lightningd/invoice.c b/lightningd/invoice.c
index f43e967522a8..5f756941089c 100644
--- a/lightningd/invoice.c
+++ b/lightningd/invoice.c
@@ -1684,8 +1684,7 @@ static void add_stub_blindedpath(const tal_t *ctx,
 	inv->invoice_blindedpay[0]->features = NULL;
 
 	/* Recalc ->fields */
-	tal_free(inv->fields);
-	inv->fields = tlv_make_fields(inv, tlv_invoice);
+	tlv_update_fields(inv, tlv_invoice, &inv->fields);
 }
 
 static struct command_result *json_createinvoice(struct command *cmd,
diff --git a/lightningd/offer.c b/lightningd/offer.c
index 0156fc808320..17a417c50ac1 100644
--- a/lightningd/offer.c
+++ b/lightningd/offer.c
@@ -467,7 +467,7 @@ static struct command_result *json_createinvoicerequest(struct command *cmd,
 	 *  [Signature Calculation](#signature-calculation) using the `invreq_payer_id`.
 	 */
 	/* This populates the ->fields from our entries */
-	invreq->fields = tlv_make_fields(invreq, tlv_invoice_request);
+	tlv_update_fields(invreq, tlv_invoice_request, &invreq->fields);
 	merkle_tlv(invreq->fields, &merkle);
 	invreq->signature = tal(invreq, struct bip340sig);
 	hsm_sign_b12(cmd->ld, "invoice_request", "signature",
diff --git a/wire/tlvstream.c b/wire/tlvstream.c
index 4813b3514947..9e1c774261bb 100644
--- a/wire/tlvstream.c
+++ b/wire/tlvstream.c
@@ -365,17 +365,30 @@ void towire_tlv(u8 **pptr,
 
 }
 
-struct tlv_field *tlv_make_fields_(const struct tlv_record_type *types,
-				   size_t num_types,
-				   const void *record)
+void tlv_update_fields_(const struct tlv_record_type *types,
+			size_t num_types,
+			const void *record,
+			struct tlv_field **tlv_fields)
 {
+	/* We merge unknown fields from record */
+	const struct tlv_field *field, *end;
 	struct tlv_field *fields = tal_arr(record, struct tlv_field, 0);
 
+	field = first_unknown_field(*tlv_fields, &end);
+
 	for (size_t i = 0; i < num_types; i++) {
 		struct tlv_field f;
 		u8 *val;
 		if (i != 0)
 			assert(types[i].type > types[i-1].type);
+
+		/* Add any unknowns which precede this. */
+		while (field && field->numtype < types[i].type) {
+			tal_steal(fields, field->value);
+			tal_arr_expand(&fields, *field);
+			field = next_unknown_field(field, end);
+		}
+
 		val = types[i].towire(NULL, record);
 		if (!val)
 			continue;
@@ -386,5 +399,14 @@ struct tlv_field *tlv_make_fields_(const struct tlv_record_type *types,
 		f.value = tal_steal(fields, val);
 		tal_arr_expand(&fields, f);
 	}
-	return fields;
+
+	/* Add any unknowns at the end. */
+	while (field) {
+		tal_steal(fields, field->value);
+		tal_arr_expand(&fields, *field);
+		field = next_unknown_field(field, end);
+	}
+
+	tal_free(*tlv_fields);
+	*tlv_fields = fields;
 }
diff --git a/wire/tlvstream.h b/wire/tlvstream.h
index 0b3f71735dba..090ba4e21433 100644
--- a/wire/tlvstream.h
+++ b/wire/tlvstream.h
@@ -30,13 +30,14 @@ struct tlv_field {
 /* Given any tlvstream serialize the raw fields (untyped ones). */
 void towire_tlvstream_raw(u8 **pptr, struct tlv_field *fields);
 
-/* Given a tlv record with manually-set fields, populate ->fields */
-#define tlv_make_fields(tlv, type)					\
-	tlv_make_fields_(tlvs_##type, TLVS_ARRAY_SIZE_##type, (tlv))
+/* Given a tlv record with manually-set fields, repopulate ->fields (maintaining any unknowns)*/
+#define tlv_update_fields(tlv, type, tlv_fields)			\
+	tlv_update_fields_(tlvs_##type, TLVS_ARRAY_SIZE_##type, (tlv), (tlv_fields))
 
-struct tlv_field *tlv_make_fields_(const struct tlv_record_type *types,
-				   size_t num_types,
-				   const void *record);
+void tlv_update_fields_(const struct tlv_record_type *types,
+			size_t num_types,
+			const void *record,
+			struct tlv_field **tlv_fields);
 
 /**
  * fromwire_tlv: generic TLV decode engine