diff --git a/devtools/onion.c b/devtools/onion.c
index 5ebca0578..5a8d53eaf 100644
--- a/devtools/onion.c
+++ b/devtools/onion.c
@@ -220,15 +220,16 @@ static void runtest(const char *filename)
 		json_to_pubkey(buffer, pubkeytok, &pubkey);
 		if (!typetok || json_tok_streq(buffer, typetok, "legacy")) {
 			/* Legacy has a single 0 prepended as "realm" byte */
-			full = tal_arrz(ctx, u8, 1);
+			full = tal_arrz(ctx, u8, 33);
+			memcpy(full + 1, payload, tal_bytelen(payload));
 		} else {
 			/* TLV has length prepended */
 			full = tal_arr(ctx, u8, 0);
 			towire_bigsize(&full, tal_bytelen(payload));
+			prepended = tal_bytelen(full);
+			tal_resize(&full, prepended + tal_bytelen(payload));
+			memcpy(full + prepended, payload, tal_bytelen(payload));
 		}
-		prepended = tal_bytelen(full);
-		tal_resize(&full, prepended + tal_bytelen(payload));
-		memcpy(full + prepended, payload, tal_bytelen(payload));
 		sphinx_add_hop(path, &pubkey, full);
 	}
 	res = create_onionpacket(ctx, path, &shared_secrets);