diff --git a/fastapi/encoders.py b/fastapi/encoders.py
index c416abbcb..0ad548b94 100644
--- a/fastapi/encoders.py
+++ b/fastapi/encoders.py
@@ -240,7 +240,8 @@ def jsonable_encoder(
             custom_encoder=encoders,
             sqlalchemy_safe=sqlalchemy_safe,
         )
-    if dataclasses.is_dataclass(type(obj)):
+    if dataclasses.is_dataclass(obj):
+        assert not isinstance(obj, type)
         obj_dict = dataclasses.asdict(obj)
         return jsonable_encoder(
             obj_dict,
diff --git a/fastapi/routing.py b/fastapi/routing.py
index 40c66f2f2..5418ad982 100644
--- a/fastapi/routing.py
+++ b/fastapi/routing.py
@@ -119,7 +119,8 @@ def _prepare_response_content(
             )
             for k, v in res.items()
         }
-    elif dataclasses.is_dataclass(type(res)):
+    elif dataclasses.is_dataclass(res):
+        assert not isinstance(res, type)
         return dataclasses.asdict(res)
     return res