diff --git a/bin/brew-upgrade b/bin/brew-upgrade
index 0e1c2ad..d7e1d6d 100755
--- a/bin/brew-upgrade
+++ b/bin/brew-upgrade
@@ -6,4 +6,4 @@ set -euf -o pipefail
 /opt/homebrew/bin/brew upgrade
 /opt/homebrew/bin/brew bundle install --file="$1" --cleanup --zap
 /opt/homebrew/bin/brew autoremove
-[[ "$2" == "--prune" ]] && /opt/homebrew/bin/brew cleanup --prune=all
+[[ "${2:-}" == "--prune" ]] && /opt/homebrew/bin/brew cleanup --prune=all