diff --git a/lib/picotls.c b/lib/picotls.c
index b1368d8..d1064e3 100644
--- a/lib/picotls.c
+++ b/lib/picotls.c
@@ -6506,7 +6506,9 @@
     if (ptls_buffer_reserve(buf, l * 2 + 1) != 0)
         return 0;
 
-    buf->off = (uint8_t *)ptls_hexdump((char *)(buf->base + buf->off), s, l) - buf->base;
+    ptls_hexdump((char *)(buf->base + buf->off), s, l);
+    buf->off += l * 2;
+
     return 1;
 }