onClick does not workon first click

I'm trying to let a button element switch its text between 'Follow' and 'Unfollow', but why isn't the javascript reacting on the first click on the button?

HTML:

<form method="post">
    <button class="follow" onclick="this.innerHTML = follow_test(this.innerHTML)">

        Follow

    </button>
</form>

JavaScript:

    function follow_test(string) {
        if (string === 'Follow') { return ('Unfollow'); } else { return('Follow'); }
    }