Fix: Allowed rule is not evaluated for fields w/ a None value

Closes #454

CHANGES.rst

@@ -30,6 +30,8 @@ Fixed
 - Defaults are applied to undefined fields from an ``allow_unknown``
   definition (`#310`_)
 - The ``forbidden`` value now handles any input type (`#449`_)
+- The `allowed` rule will not be evaluated on fields that have a legit ``None``
+  value (`#454`_)
 
 Improved
 ~~~~~~~~
@@ -62,6 +64,7 @@ Docs
 - Add a few clarifications to the GitHub issue template
 - Update README link; make it point to the new PyPI website
 
+.. _`#454`: https://github.com/pyeve/cerberus/pull/454
 .. _`#451`: https://github.com/pyeve/cerberus/pull/451
 .. _`#449`: https://github.com/pyeve/cerberus/pull/449
 .. _`#435`: https://github.com/pyeve/cerberus/pull/435

cerberus/tests/test_validation.py

@@ -75,6 +75,11 @@ def test_nullable_field():
     assert_fail({'a_not_nullable_field_without_type': None})
 
 
+def test_nullable_skips_allowed():
+    schema = {'role': {'allowed': ['agent', 'client', 'supplier'], 'nullable': True}}
+    assert_success({'role': None}, schema)
+
+
 def test_readonly_field():
     field = 'a_readonly_string'
     assert_fail(

cerberus/validator.py

@@ -1326,6 +1326,7 @@ def _validate_nullable(self, nullable, field, value):
             if not nullable:
                 self._error(field, errors.NOT_NULLABLE)
             self._drop_remaining_rules(
+                'allowed',
                 'empty',
                 'forbidden',
                 'items',