HKDF-Expand-Label being used is that of RFC 8446, hence uses the "tls13 " prefix
diff --git a/lib/picotls.c b/lib/picotls.c
index d817b01..a7ed1b7 100644
--- a/lib/picotls.c
+++ b/lib/picotls.c
@@ -1140,7 +1140,7 @@
sched->hashes[0].ctx->final(sched->hashes[0].ctx, transcript_hash, PTLS_HASH_FINAL_MODE_SNAPSHOT);
if ((ret = ptls_hkdf_expand_label(sched->hashes[0].algo, dst, 8, ptls_iovec_init(secret, sched->hashes[0].algo->digest_size),
- label, ptls_iovec_init(transcript_hash, sched->hashes[0].algo->digest_size), "")) != 0)
+ label, ptls_iovec_init(transcript_hash, sched->hashes[0].algo->digest_size), NULL)) != 0)
goto Exit;
Exit:
