diff --git a/websockify/websocket.py b/websockify/websocket.py
index dae93b44..af87d3e6 100644
--- a/websockify/websocket.py
+++ b/websockify/websocket.py
@@ -731,7 +731,7 @@ def _sendmsg(self, opcode, msg):
         if self.client:
             mask = b''
             for i in range(4):
-                mask += random.randrange(256)
+                mask += random.randrange(256).to_bytes()
             frame = self._encode_hybi(opcode, msg, mask)
         else:
             frame = self._encode_hybi(opcode, msg)