diff --git a/buildbot_nix/buildbot_nix/build_canceller.py b/buildbot_nix/buildbot_nix/build_canceller.py
index d020ce9fd..79c7e008a 100644
--- a/buildbot_nix/buildbot_nix/build_canceller.py
+++ b/buildbot_nix/buildbot_nix/build_canceller.py
@@ -10,7 +10,7 @@
 
 def branch_key_for_pr(build: dict[str, Any]) -> str:
     """Extract a unique key for PR/change to cancel old builds."""
-    branch = build.get("branch", "")
+    branch = build.get("branch") or ""
 
     # For GitHub/Gitea/GitLab PRs
     if branch.startswith(("refs/pull/", "refs/merge-requests/")):