diff --git a/little_boxes/activitypub.py b/little_boxes/activitypub.py
index 0902648..8cae950 100644
--- a/little_boxes/activitypub.py
+++ b/little_boxes/activitypub.py
@@ -24,7 +24,7 @@ from typing import Type
 
 logger = logging.getLogger(__name__)
 
-UninitializedBackendError = Error('a backend must be initialized')
+UninitializedBackendError = Error("a backend must be initialized")
 
 # Helper/shortcut for typing
 ObjectType = Dict[str, Any]
@@ -591,7 +591,7 @@ class Follow(BaseActivity):
         # XXX The new_following event will be triggered by Accept
         pass
 
-    def _undo_inbox(self, as_actor: 'Person') -> None:
+    def _undo_inbox(self, as_actor: "Person") -> None:
         if BACKEND is None:
             raise UninitializedBackendError
 
diff --git a/little_boxes/backend.py b/little_boxes/backend.py
index 04e62b9..3ddcad3 100644
--- a/little_boxes/backend.py
+++ b/little_boxes/backend.py
@@ -2,7 +2,7 @@ import abc
 import typing
 
 if typing.TYPE_CHECKING:
-    from little_boxes import activitypub as ap
+    from little_boxes import activitypub as ap  # noqa: type checking
 
 
 class Backend(abc.ABC):