diff --git a/storage.py b/storage.py
index f1c5468..0ca4523 100644
--- a/storage.py
+++ b/storage.py
@@ -4,21 +4,22 @@
     Shamelessly inspired from Syndace's python-omemo examples.
 """
 
-import omemo
 
 import os
 import copy
 import json
-from typing import Any, Dict, Set, Union
+from typing import Any, Dict, List, Set, Union
+
+import omemo
 
 
 class SyncFileStorage(omemo.Storage):
     def __init__(self, storage_dir: str) -> None:
         self.storage_dir = storage_dir
         self.__state = None
-        self.__own_data = None  # type: Union[None, Dict[str, str]]
+        self.__own_data = None  # type: Union[None, Dict[str, Union[str, int]]]
         self.__sessions = {}  # type: Dict[str, Dict[int, Any]]
-        self.__devices = {}  # type: Dict[str, Dict[str, Set[int]]]
+        self.__devices = {}  # type: Dict[str, Dict[str, List[int]]]
 
     def dump(self):
         return copy.deepcopy({
@@ -72,8 +73,8 @@ class SyncFileStorage(omemo.Storage):
         self.__sessions[bare_jid] = self.__sessions.get(bare_jid, {})
         self.__sessions[bare_jid][device_id] = session
 
-    def loadActiveDevices(self, _callback, bare_jid: str) -> Union[None, Set[int]]:
-        if self.__devices is None:
+    def loadActiveDevices(self, _callback, bare_jid: str) -> Union[None, List[int]]:
+        if not self.__devices:
             try:
                 filepath = os.path.join(self.storage_dir, 'devices.json')
                 with open(filepath, 'r') as f:
@@ -91,8 +92,8 @@ class SyncFileStorage(omemo.Storage):
         with open(filepath, 'w') as f:
             json.dump(self.__devices, f)
 
-    def loadInactiveDevices(self, _callback, bare_jid: str) -> Union[None, Set[int]]:
-        if self.__devices is None:
+    def loadInactiveDevices(self, _callback, bare_jid: str) -> Union[None, List[int]]:
+        if not self.__devices:
             try:
                 filepath = os.path.join(self.storage_dir, 'devices.json')
                 with open(filepath, 'r') as f: