diff --git a/server/atheneum/service/transformation_service.py b/server/atheneum/service/transformation_service.py
index 774c0ad..20eaee6 100644
--- a/server/atheneum/service/transformation_service.py
+++ b/server/atheneum/service/transformation_service.py
@@ -77,7 +77,7 @@ def register_transformer(
     if model_name not in _model_transformers:
         _model_transformers[model_name] = model_serializer
     else:
-        raise ValueError(
+        raise KeyError(
             ' '.join([
                 'A transformer for type "{}" already exists with class "{}".',
                 'Cannot register a new transformer with class "{}"'
@@ -100,12 +100,12 @@ def serialize_model(model_obj: db.Model,
 
 
 def deserialize_model(
-        model_type: str,
+        model_type: Type[db.Model],
         json_model_object: dict,
         options: Optional[List[str]] = None) -> db.Model:
     """Lookup a Model and hand it off to the deserializer."""
     try:
-        transformer = _model_transformers[model_type]
+        transformer = _model_transformers[model_type.__name__]
         return transformer(
             transformer.type()).deserialize(json_model_object, options)
     except KeyError:
diff --git a/server/tests/service/test_transformation_service.py b/server/tests/service/test_transformation_service.py
index 297cc17..4452733 100644
--- a/server/tests/service/test_transformation_service.py
+++ b/server/tests/service/test_transformation_service.py
@@ -24,7 +24,7 @@ def test_deserialize_model():
     user_token_json = {
         'token': 'test'
     }
-    result = deserialize_model('UserToken', user_token_json)
+    result = deserialize_model(UserToken, user_token_json)
     assert result is not None
     assert result.token == 'test'
 
@@ -40,7 +40,7 @@ def test_registering_two_transformers_of_the_same_type():
             pass
 
     error = None
-    with pytest.raises(ValueError) as e_info:
+    with pytest.raises(KeyError) as e_info:
         error = e_info
         register_transformer(BadTransformer)