diff --git a/nixprstatus/pr.py b/nixprstatus/pr.py
index 6be07a8..66c169f 100644
--- a/nixprstatus/pr.py
+++ b/nixprstatus/pr.py
@@ -73,7 +73,7 @@ def get_pr(pr: int) -> dict[str, Any]:
 
 
 def pr_merge_status(
-    pr: int, branches: list[str] = DEFAULT_BRANCHES, check_backport: bool = True
+    pr: int, branches: list[str] | None = DEFAULT_BRANCHES, check_backport: bool = True
 ) -> PRStatus:
     if not branches:
         branches = DEFAULT_BRANCHES