diff --git a/yacs/config.py b/yacs/config.py
index 54bec35..17400a1 100644
--- a/yacs/config.py
+++ b/yacs/config.py
@@ -45,7 +45,7 @@
     _FILE_TYPES = (io.IOBase,)
 
 # CfgNodes can only contain a limited set of valid types
-_VALID_TYPES = {tuple, list, str, int, float, bool}
+_VALID_TYPES = {tuple, list, str, int, float, bool, type(None)}
 # py2 allow for str and unicode
 if _PY2:
     _VALID_TYPES = _VALID_TYPES.union({unicode})  # noqa: F821
@@ -489,6 +489,12 @@ def _check_and_coerce_cfg_value_type(replacement, original, key, full_key):
     if replacement_type == original_type:
         return replacement
 
+    # If either of them is None, allow type conversion to one of the valid types
+    if (replacement_type == type(None) and original_type in _VALID_TYPES) or (
+        original_type == type(None) and replacement_type in _VALID_TYPES
+    ):
+        return replacement
+
     # Cast replacement from from_type to to_type if the replacement and original
     # types match from_type and to_type
     def conditional_cast(from_type, to_type):
diff --git a/yacs/tests.py b/yacs/tests.py
index 6748214..23bc871 100644
--- a/yacs/tests.py
+++ b/yacs/tests.py
@@ -3,6 +3,7 @@
 import unittest
 
 import yacs.config
+import yaml
 from yacs.config import CfgNode as CN
 
 try:
@@ -196,8 +197,14 @@ def test_nonexistant_key_from_list(self):
             cfg.merge_from_list(opts)
 
     def test_load_cfg_invalid_type(self):
-        # FOO.BAR.QUUX will have type None, which is not allowed
-        cfg_string = "FOO:\n BAR:\n  QUUX:"
+        class CustomClass(yaml.YAMLObject):
+            """A custom class that yaml.safe_load can load."""
+
+            yaml_loader = yaml.SafeLoader
+            yaml_tag = u"!CustomClass"
+
+        # FOO.BAR.QUUX will have type CustomClass, which is not allowed
+        cfg_string = "FOO:\n BAR:\n  QUUX: !CustomClass {}"
         with self.assertRaises(AssertionError):
             yacs.config.load_cfg(cfg_string)