diff --git a/pytorch3d/implicitron/dataset/json_index_dataset_map_provider.py b/pytorch3d/implicitron/dataset/json_index_dataset_map_provider.py
index d4f628c3..35ee7567 100644
--- a/pytorch3d/implicitron/dataset/json_index_dataset_map_provider.py
+++ b/pytorch3d/implicitron/dataset/json_index_dataset_map_provider.py
@@ -113,9 +113,8 @@ class JsonIndexDatasetMapProvider(DatasetMapProviderBase):  # pyre-ignore [13]
         Called by get_default_args(JsonIndexDatasetMapProvider) to
         not expose certain fields of each dataset class.
         """
-        with open_dict(args):
-            for key in _NEED_CONTROL:
-                del args[key]
+        for key in _NEED_CONTROL:
+            del args[key]
 
     def create_dataset(self):
         """
diff --git a/pytorch3d/implicitron/dataset/json_index_dataset_map_provider_v2.py b/pytorch3d/implicitron/dataset/json_index_dataset_map_provider_v2.py
index 29027ec6..a5257218 100644
--- a/pytorch3d/implicitron/dataset/json_index_dataset_map_provider_v2.py
+++ b/pytorch3d/implicitron/dataset/json_index_dataset_map_provider_v2.py
@@ -300,9 +300,8 @@ class JsonIndexDatasetMapProviderV2(DatasetMapProviderBase):  # pyre-ignore [13]
         Called by get_default_args(JsonIndexDatasetMapProviderV2) to
         not expose certain fields of each dataset class.
         """
-        with open_dict(args):
-            for key in _NEED_CONTROL:
-                del args[key]
+        for key in _NEED_CONTROL:
+            del args[key]
 
     def create_dataset(self):
         # The dataset object is created inside `self.get_dataset_map`
diff --git a/pytorch3d/implicitron/tools/config.py b/pytorch3d/implicitron/tools/config.py
index 1605f8b4..9fb7f6c7 100644
--- a/pytorch3d/implicitron/tools/config.py
+++ b/pytorch3d/implicitron/tools/config.py
@@ -881,7 +881,8 @@ def get_default_args_field(
     def create():
         args = get_default_args(C, _do_not_process=_do_not_process)
         if _hook is not None:
-            _hook(args)
+            with open_dict(args):
+                _hook(args)
         return args
 
     return dataclasses.field(default_factory=create)
@@ -915,7 +916,8 @@ def _get_default_args_field_from_registry(
         C = registry.get(base_class_wanted=base_class_wanted, name=name)
         args = get_default_args(C, _do_not_process=_do_not_process)
         if _hook is not None:
-            _hook(args)
+            with open_dict(args):
+                _hook(args)
         return args
 
     return dataclasses.field(default_factory=create)
diff --git a/tests/implicitron/test_config.py b/tests/implicitron/test_config.py
index 590b9dea..56e7ecab 100644
--- a/tests/implicitron/test_config.py
+++ b/tests/implicitron/test_config.py
@@ -691,12 +691,17 @@ class TestConfig(unittest.TestCase):
             fruit2_class_type: str = "Pear"
             a: A
             a2: A
+            a3: A
 
             @classmethod
             def a_tweak_args(cls, type, args):
                 assert type == A
                 args.n = 993
 
+            @classmethod
+            def a3_tweak_args(cls, type, args):
+                del args["n"]
+
             @classmethod
             def fruit_tweak_args(cls, type, args):
                 assert issubclass(type, Fruit)
@@ -707,6 +712,7 @@ class TestConfig(unittest.TestCase):
         args = get_default_args(Wrapper)
         self.assertEqual(args.a_args.n, 993)
         self.assertEqual(args.a2_args.n, 9)
+        self.assertEqual(args.a3_args, {})
         self.assertEqual(args.fruit_Pear_args.n_pips, 19)
         self.assertEqual(args.fruit2_Pear_args.n_pips, 13)