diff --git a/pytest_aiohttp/plugin.py b/pytest_aiohttp/plugin.py
index 68c8ad8..31b26fa 100644
--- a/pytest_aiohttp/plugin.py
+++ b/pytest_aiohttp/plugin.py
@@ -160,7 +160,7 @@ async def go(
         elif isinstance(__param, BaseTestServer):
             client = aiohttp_client_cls(__param, **kwargs)
         else:
-            raise ValueError("Unknown argument type: %r" % type(__param))
+            raise ValueError(f"Unknown argument type: {type(__param)!r}")
 
         await client.start_server()
         clients.append(client)