Submit button not taking class

I have the following set in place for Contact Form 7, on my WordPress 5.0 setup…

<label> Your Name (required)
    [text* your-name class:required "i.e. John Learr"] </label>
    
<label> Your Email (required)
    [email* your-email] </label>

<label> Subject
    [text* your-subject] </label>

<label> Your Message
    [textarea* your-message] </label>

[submit "Send" class:nicksubmit]

…but the DOM is loading the submit button as the following….

<input type="submit" value="Send" class="wpcf7-form-control wpcf7-submit">

It should be adding nicksubmit as a class on that element, but it’s not.