diff --git a/radicale/hook/__init__.py b/radicale/hook/__init__.py
index a1aea80f..98562bcb 100644
--- a/radicale/hook/__init__.py
+++ b/radicale/hook/__init__.py
@@ -40,4 +40,9 @@ class HookNotificationItem:
         self.content = content
 
     def to_json(self):
-        return json.dumps(self, default=lambda o: o.__dict__, sort_keys=True, indent=4)
+        return json.dumps(
+            self,
+            default=lambda o: o.__dict__,
+            sort_keys=True,
+            indent=4
+        )