diff --git a/bin/git-coauthor b/bin/git-coauthor
index 522f2db..c51400e 100755
--- a/bin/git-coauthor
+++ b/bin/git-coauthor
@@ -9,7 +9,7 @@ test -z "$coauthor" && echo "co-author required." 1>&2 && exit 1
 test -z "$email" && echo "co-author email required." 1>&2 && exit 1
 
 old_message="$(git log --format=%B -n1)"
-if [[ $old_message == *"Co-authored-by:"* ]]; then
+if [[ "$old_message" == *"Co-authored-by:"* ]]; then
   git commit --amend -m "$old_message
 Co-authored-by: $coauthor <$email>"
 else