diff --git a/messenger.py b/messenger.py
index 88c2333..40ed3e8 100644
--- a/messenger.py
+++ b/messenger.py
@@ -58,7 +58,7 @@ def CONCAT(ad, header):
     header_tup = pickle.dumps(
         (header.dh.public_bytes(serialization.Encoding.X962,serialization.PublicFormat.UncompressedPoint), header.pn, header.n),
     )
-    return len(ad).to_bytes(8) + ad + header_tup
+    return len(ad).to_bytes(8, "big") + ad + header_tup
 
 class MessengerServer:
     def __init__(self, server_signing_key, server_decryption_key):