--- a/buggy.py
+++ b/buggy.py
@@ -1,3 +1,5 @@
-def append_event(value: str, events: list[str] = []) -> list[str]:
+def append_event(value: str, events: list[str] | None = None) -> list[str]:
+    if events is None:
+        events = []
     events.append(value)
     return events
