diff --git a/fastapi/routing.py b/fastapi/routing.py
index 142286d55..05e9b0bc1 100644
--- a/fastapi/routing.py
+++ b/fastapi/routing.py
@@ -188,9 +188,6 @@ def _prepare_response_content(
             )
             for k, v in res.items()
         }
-    elif dataclasses.is_dataclass(res):
-        assert not isinstance(res, type)
-        return dataclasses.asdict(res)
     return res