diff --git a/pytorch3d/implicitron/tools/config.py b/pytorch3d/implicitron/tools/config.py
index 5e7cb2bf..3c2c4d88 100644
--- a/pytorch3d/implicitron/tools/config.py
+++ b/pytorch3d/implicitron/tools/config.py
@@ -746,7 +746,8 @@ def _get_type_to_process(type_) -> Optional[Tuple[Type, _ProcessType]]:
             return
         underlying = args[0] if args[1] is type(None) else args[1]  # noqa: E721
         if (
-            issubclass(underlying, ReplaceableBase)
+            isinstance(underlying, type)
+            and issubclass(underlying, ReplaceableBase)
             and ReplaceableBase in underlying.__bases__
         ):
             return underlying, _ProcessType.OPTIONAL_REPLACEABLE
diff --git a/tests/implicitron/test_config.py b/tests/implicitron/test_config.py
index fc3ccde0..6f238412 100644
--- a/tests/implicitron/test_config.py
+++ b/tests/implicitron/test_config.py
@@ -109,6 +109,7 @@ class TestConfig(unittest.TestCase):
         self.assertIsNone(gt(Optional[MainTest]))
         self.assertIsNone(gt(Tuple[Fruit]))
         self.assertIsNone(gt(Tuple[Fruit, Animal]))
+        self.assertIsNone(gt(Optional[List[int]]))
 
     def test_simple_replacement(self):
         struct = get_default_args(MainTest)