diff --git a/conservancy_beancount/plugin/core.py b/conservancy_beancount/plugin/core.py
index 39dc7e2..f2fb69b 100644
--- a/conservancy_beancount/plugin/core.py
+++ b/conservancy_beancount/plugin/core.py
@@ -23,6 +23,7 @@ from .. import errors as errormod
 
 from typing import (
     Any,
+    Dict,
     FrozenSet,
     Generic,
     Iterable,
@@ -113,7 +114,7 @@ class MetadataEnum:
     def __init__(self,
                  key: MetaKey,
                  standard_values: Iterable[MetaValueEnum],
-                 aliases_map: Mapping[MetaValueEnum, MetaValueEnum],
+                 aliases_map: Optional[Mapping[MetaValueEnum, MetaValueEnum]]=None,
     ) -> None:
         """Specify allowed values and aliases for this metadata.
 
@@ -129,9 +130,9 @@ class MetadataEnum:
         """
         self.key = key
         self._stdvalues = frozenset(standard_values)
-        self._aliases = dict(aliases_map)
+        self._aliases: Dict[MetaValueEnum, MetaValueEnum] = dict(aliases_map or ())
+        assert self._stdvalues.issuperset(self._aliases.values())
         self._aliases.update((v, v) for v in standard_values)
-        assert self._stdvalues == set(self._aliases.values())
 
     def __repr__(self) -> str:
         return "{}<{}>".format(type(self).__name__, self.key)
diff --git a/conservancy_beancount/plugin/meta_tax_implication.py b/conservancy_beancount/plugin/meta_tax_implication.py
index 3f0a61a..2716dd1 100644
--- a/conservancy_beancount/plugin/meta_tax_implication.py
+++ b/conservancy_beancount/plugin/meta_tax_implication.py
@@ -43,7 +43,7 @@ class MetaTaxImplication(core._NormalizePostingMetadataHook):
         'USA-Corporation',
         'USA-LLC-No-1099',
         'W2',
-    ], {})
+    ])
 
     def _run_on_post(self, txn: Transaction, post: data.Posting) -> bool:
         return bool(