diff --git a/mitmproxy/addons/script.py b/mitmproxy/addons/script.py
index 3ac4baa28..4e146d67d 100644
--- a/mitmproxy/addons/script.py
+++ b/mitmproxy/addons/script.py
@@ -29,6 +29,7 @@ def load_script(path: str) -> typing.Optional[types.ModuleType]:
     try:
         loader = importlib.machinery.SourceFileLoader(fullname, path)
         spec = importlib.util.spec_from_loader(fullname, loader=loader)
+        assert spec
         m = importlib.util.module_from_spec(spec)
         loader.exec_module(m)
         if not getattr(m, "name", None):
diff --git a/mitmproxy/platform/windows.py b/mitmproxy/platform/windows.py
index 46047a954..b447a7517 100644
--- a/mitmproxy/platform/windows.py
+++ b/mitmproxy/platform/windows.py
@@ -17,6 +17,12 @@ import collections.abc
 import pydivert
 import pydivert.consts
 
+if typing.TYPE_CHECKING:
+    class WindowsError(OSError):
+        @property
+        def winerror(self) -> int:
+            return 42
+
 REDIRECT_API_HOST = "127.0.0.1"
 REDIRECT_API_PORT = 8085
 
diff --git a/tox.ini b/tox.ini
index 591e25c58..8963dcf6f 100644
--- a/tox.ini
+++ b/tox.ini
@@ -28,7 +28,14 @@ commands =
   python ./test/filename_matching.py
 
 [testenv:mypy]
-deps = mypy==0.812
+deps =
+  mypy==0.902
+  types-certifi==0.1.3
+  types-Flask==1.1.0
+  types-Werkzeug==1.0.1
+  types-requests==0.1.11
+  types-cryptography==3.3.2
+
 commands =
   mypy {posargs}