diff --git a/adb/adb_protocol.py b/adb/adb_protocol.py
index 7fd2552..684f151 100644
--- a/adb/adb_protocol.py
+++ b/adb/adb_protocol.py
@@ -579,7 +579,7 @@
       # user.
       msg = _AdbMessage.Make(
           b'AUTH', _AdbMessageHeader.AUTH_RSAPUBLICKEY, 0,
-          self._rsa_keys[0].GetPublicKey() + '\0')
+          self._rsa_keys[0].GetPublicKey() + b'\0')
       msg.Write(self._usb)
       try:
         reply = _AdbMessage.Read(self._usb, self._auth_timeout_ms)