Make dropdownlist mandatory for continuing

I have a form for users to fill out various inputs and dropdowns. One of the dropdowns has a yes or no boolean as the options. I'm trying to write an if statement to prevent users from leaving it blank.

<div class="form-group col-xs-12 col-sm-4">
        <div class="input-group ">
            <span class="input-group-addon white">Yes or No</span>
            <select class="form-control gray" id="returnSelect">
                <option value="1">Yes</option>
                <option value="0">No</option>
            </select>
        </div>
    </div>

I've tried the following:

        if (data.yesOrNo == null)
        {
            return BadRequest("Please select 'Yes' or 'No' for Multiple Returns");
        }

and

        if (data.yesOrNo != true && data.yesOrNo != false)
        {
            return BadRequest("Please select 'Yes' or 'No' for Multiple Returns");
        }
    

A bit primitive, but neither work. Where am I going wrong?