diff --git a/pytorch3d/implicitron/tools/config.py b/pytorch3d/implicitron/tools/config.py
index cf9970b6..eaf91151 100644
--- a/pytorch3d/implicitron/tools/config.py
+++ b/pytorch3d/implicitron/tools/config.py
@@ -611,6 +611,9 @@ def _params_iter(C):
 
 
 def _is_immutable_type(type_: Type, val: Any) -> bool:
+    if val is None:
+        return True
+
     PRIMITIVE_TYPES = (int, float, bool, str, bytes, tuple)
     # sometimes type can be too relaxed (e.g. Any), so we also check values
     if isinstance(val, PRIMITIVE_TYPES):
diff --git a/tests/implicitron/test_config.py b/tests/implicitron/test_config.py
index 2511566f..2f591edd 100644
--- a/tests/implicitron/test_config.py
+++ b/tests/implicitron/test_config.py
@@ -9,7 +9,7 @@ import textwrap
 import unittest
 from dataclasses import dataclass, field, is_dataclass
 from enum import Enum
-from typing import Any, List, Optional, Set, Tuple
+from typing import Any, Dict, List, Optional, Set, Tuple
 
 from omegaconf import DictConfig, ListConfig, OmegaConf, ValidationError
 from pytorch3d.implicitron.tools.config import (
@@ -735,6 +735,7 @@ class MockDataclass:
     field_no_default: int
     field_primitive_type: int = 42
     field_optional_none: Optional[int] = None
+    field_optional_dict_none: Optional[Dict] = None
     field_optional_with_value: Optional[int] = 42
     field_list_type: List[int] = field(default_factory=lambda: [])
 
@@ -753,6 +754,7 @@ class MockClassWithInit:  # noqa: B903
         field_no_default: int,
         field_primitive_type: int = 42,
         field_optional_none: Optional[int] = None,
+        field_optional_dict_none: Optional[Dict] = None,
         field_optional_with_value: Optional[int] = 42,
         field_list_type: List[int] = [],  # noqa: B006
         field_reference_type: RefObject = REF_OBJECT,
@@ -761,6 +763,7 @@ class MockClassWithInit:  # noqa: B903
         self.field_no_default = field_no_default
         self.field_primitive_type = field_primitive_type
         self.field_optional_none = field_optional_none
+        self.field_optional_dict_none = field_optional_dict_none
         self.field_optional_with_value = field_optional_with_value
         self.field_list_type = field_list_type
         self.field_reference_type = field_reference_type
@@ -785,8 +788,18 @@ class TestRawClasses(unittest.TestCase):
             self.assertNotIn("field_no_default", dataclass_defaults)
             self.assertNotIn("field_no_nothing", dataclass_defaults)
             self.assertNotIn("field_reference_type", dataclass_defaults)
+            expected_defaults = [
+                "field_primitive_type",
+                "field_optional_none",
+                "field_optional_dict_none",
+                "field_optional_with_value",
+                "field_list_type",
+            ]
+
             if cls == MockDataclass:  # we don't remove undefaulted from dataclasses
                 dataclass_defaults.field_no_default = 0
+                expected_defaults.insert(0, "field_no_default")
+            self.assertEqual(list(dataclass_defaults), expected_defaults)
             for name, val in dataclass_defaults.items():
                 self.assertTrue(hasattr(self._instances[cls], name))
                 self.assertEqual(val, getattr(self._instances[cls], name))