diff --git a/shadowsocks/encrypt.py b/shadowsocks/encrypt.py
index 91f7439..336f455 100644
--- a/shadowsocks/encrypt.py
+++ b/shadowsocks/encrypt.py
@@ -204,4 +204,4 @@ def encrypt_all(password, method, op, data):
             data = data[iv_len:]
         cipher = m(method, key, iv, op)
         result.append(cipher.update(data))
-        return ''.join(result)
+        return b''.join(result)
diff --git a/shadowsocks/udprelay.py b/shadowsocks/udprelay.py
index b4951be..2b8b12f 100644
--- a/shadowsocks/udprelay.py
+++ b/shadowsocks/udprelay.py
@@ -229,7 +229,7 @@ class UDPRelay(object):
             if header_result is None:
                 return
             # addrtype, dest_addr, dest_port, header_length = header_result
-            response = '\x00\x00\x00' + data
+            response = b'\x00\x00\x00' + data
         client_addr = self._client_fd_to_server_addr.get(sock.fileno())
         if client_addr:
             self._server_socket.sendto(response, client_addr)