diff --git a/fastapi/encoders.py b/fastapi/encoders.py
index 451ea0760..0ad548b94 100644
--- a/fastapi/encoders.py
+++ b/fastapi/encoders.py
@@ -241,6 +241,7 @@ def jsonable_encoder(
             sqlalchemy_safe=sqlalchemy_safe,
         )
     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 54c75a027..5418ad982 100644
--- a/fastapi/routing.py
+++ b/fastapi/routing.py
@@ -120,6 +120,7 @@ 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
 
diff --git a/requirements-tests.txt b/requirements-tests.txt
index 7c1fd12f7..e87a42162 100644
--- a/requirements-tests.txt
+++ b/requirements-tests.txt
@@ -2,7 +2,7 @@
 -r requirements-docs-tests.txt
 pytest >=7.1.3,<9.0.0
 coverage[toml] >= 6.5.0,< 8.0
-mypy ==1.8.0
+mypy ==1.14.1
 dirty-equals ==0.9.0
 sqlmodel==0.0.24
 flask >=1.1.2,<4.0.0