diff --git a/bin/git-check-assertions b/bin/git-check-assertions
index 4ebc513..53fc277 100755
--- a/bin/git-check-assertions
+++ b/bin/git-check-assertions
@@ -64,7 +64,15 @@ export -f run assert_success assert_failure assert_output
 
 # main flow
 orig_ref="$(git symbolic-ref --quiet --short HEAD || git rev-parse HEAD)"
-base="$(git merge-base main HEAD)"
+if git show-ref --verify --quiet refs/heads/main; then
+  base_branch=main
+elif git show-ref --verify --quiet refs/heads/master; then
+  base_branch=master
+else
+  echo "No main or master branch found." >&2
+  exit 1
+fi
+base="$(git merge-base "$base_branch" HEAD)"
 mapfile -t commits < <(git rev-list --reverse "${base}..HEAD")
 
 echo "Base: $base"