Linear Stepper - server side validation for individual steps

Hi,

I'm trying to implement server side validation for each step in my linear stepper:

<div>
    <ul class="stepper linear">
        <li class="step">
            <div class="step-title">Step 1</div>
            <div class="step-new-content">
                <form action="..." method="post">
                     ... contents to validate ...
                </form>
                <div class="step-actions">
                    <button class="btn btn-dark btn-sm next-step" data-feedback="validationFunction">CONTINUE</button>
                </div>
            </div>
        </li>
    </ul>

In validationFunction I want to send the form via ajax request to validate the contents of an individual step and then proceed or cancel, depending on the reply.

But I never get to the above function and the flow breaks down because of the error in steppers.js (steppers.min.js): TypeError: Cannot read property 'settings' of undefined (line 425 of steppers.js)