diff --git a/fimfarchive/fetchers.py b/fimfarchive/fetchers.py
index e726b89..780e6b6 100644
--- a/fimfarchive/fetchers.py
+++ b/fimfarchive/fetchers.py
@@ -27,12 +27,13 @@ import gc
 import json
 from copy import deepcopy
 from io import BytesIO
+from typing import FrozenSet
 from zipfile import ZipFile, BadZipFile
 
 import requests
 
 from fimfarchive.exceptions import InvalidStoryError, StorySourceError
-from fimfarchive.flavors import StorySource, DataFormat, MetaPurity
+from fimfarchive.flavors import Flavor, StorySource, DataFormat, MetaPurity
 from fimfarchive.stories import Story
 
 
@@ -43,6 +44,7 @@ __all__ = (
 )
 
 
+FlavorSet = FrozenSet[Flavor]
 StreamReader = codecs.getreader('utf-8')
 
 
@@ -53,7 +55,7 @@ class Fetcher:
     prefetch_meta = False
     prefetch_data = False
 
-    flavors = frozenset()
+    flavors = frozenset()  # type: FlavorSet
 
     def __enter__(self):
         """
diff --git a/fimfarchive/utils.py b/fimfarchive/utils.py
index 648c2c0..77c0a6f 100644
--- a/fimfarchive/utils.py
+++ b/fimfarchive/utils.py
@@ -25,7 +25,7 @@ Various utilities.
 import json
 import os
 import shutil
-from collections import UserDict
+from typing import Dict, Any
 
 
 __all__ = (
@@ -52,7 +52,7 @@ class Empty(metaclass=EmptyMeta):
         return False
 
 
-class PersistedDict(UserDict):
+class PersistedDict(Dict[str, Any]):
     """
     Dictionary for simple persistance.
     """
@@ -75,22 +75,19 @@ class PersistedDict(UserDict):
         """
         Loads data from file as JSON.
         """
+        self.clear()
+        self.update(self.default)
+
         if os.path.exists(self.path):
             with open(self.path, 'rt') as fobj:
-                self.data = json.load(fobj)
-        else:
-            self.data = dict()
-
-        for k, v in self.default.items():
-            if k not in self.data:
-                self.data[k] = v
+                self.update(json.load(fobj))
 
     def save(self):
         """
         Saves data to file as JSON.
         """
         content = json.dumps(
-            self.data,
+            self,
             indent=4,
             ensure_ascii=False,
             sort_keys=True,
diff --git a/tox.ini b/tox.ini
index a2680f1..e19cc5a 100644
--- a/tox.ini
+++ b/tox.ini
@@ -10,10 +10,12 @@ python =
 commands =
     py.test
     flake8 fimfarchive tests
+    mypy --ignore-missing-imports fimfarchive
 deps =
     -rrequirements.txt
     pytest
     flake8
+    mypy
 
 [pytest]
 addopts =