“Union” value not save in Select field in CF7

When we have added the “Union” value in contact form 7 Select field and save the Form it’s showing 404 error and page are not saved.

Check this Code:-
Not working Code:-
[select menu-779 “Union” “Tester-1” “tester-123”]
Then Contact form 7 Show 404 error when you will save the Form.

When we will remove the “Union” from value then it’s Save:
Working Code:-
[select menu-779 “Tester-1” “tester-123”]

Please suggest how to edit “Union” in this Fields

Thanks