diff --git a/tanco/driver.py b/tanco/driver.py
index 6bc47e7..d6438f8 100644
--- a/tanco/driver.py
+++ b/tanco/driver.py
@@ -292,6 +292,7 @@ async def ws_talk(self, ws: w.WebSocketCommonProtocol):
         self.ensure_target()
 
         async for msg in ws:
+            assert isinstance(msg, str)
             if self.end_cmd in msg:
                 await ws.send("WARNING: ';' in message, ignoring")
                 continue