diff --git a/swh/__init__.py b/swh/__init__.py
--- a/swh/__init__.py
+++ b/swh/__init__.py
@@ -1,4 +1,3 @@
 from pkgutil import extend_path
-from typing import List
 
-__path__: List[str] = extend_path(__path__, __name__)
+__path__ = extend_path(__path__, __name__)
diff --git a/swh/storage/cassandra/storage.py b/swh/storage/cassandra/storage.py
--- a/swh/storage/cassandra/storage.py
+++ b/swh/storage/cassandra/storage.py
@@ -530,9 +530,11 @@
                 if dentry.target == content.sha1_git:
                     break
             else:
+                target = ret["target"]
+                assert target is not None
                 tokens = list(
                     self._cql_runner.skipped_content_get_tokens_from_single_hash(
-                        "sha1_git", ret["target"]
+                        "sha1_git", target
                     )
                 )
                 if tokens:
diff --git a/tox.ini b/tox.ini
--- a/tox.ini
+++ b/tox.ini
@@ -38,7 +38,7 @@
 extras =
   testing
 deps =
-  mypy
+  mypy==0.920
 commands =
   mypy swh