How to validate a hidden field

I’m using the bootstrapValidator plugin and have a hidden field whose value is set dynamically. When the user makes choices which get added to the hidden field the validation is not picking it up.

Here is the field I’m trying to validate
customerVehicles


<div class="form-group">
      <label  class="" for="fname"></label>
      <input type="hidden" name="customerVehicles" class="customerVehicles form-control" id="customerVehicles"   />
</div><!-- close form group -->

Here is my validation block


$('#step2').bootstrapValidator({
  message: 'This value is not valid',
  live: 'enabled',
  feedbackIcons: {
      valid: 'glyphicon glyphicon-ok',
      invalid: 'glyphicon glyphicon-remove',
      validating: 'glyphicon glyphicon-refresh'
  },
  submitHandler: function(validator, form, submitButton) {

      document.getElementById("next2").disabled=true;

      $.ajax({
        type: "POST",
        url: "scripts/process-step2.php",
        data: $('#step2').serialize(),
        success: function(msg){

               ...
        },
        error: function(){
          alert("error");
        }
      });//close ajax
  },
  excluded: ':disabled',
  fields: {
      coverage: {
          validators: {
              notEmpty: {
                  message: 'Please select level of coverage'
              }
          }
      },
      customerVehicles: {
           message: 'Please Make a Selection',
           validators: {
              notEmpty: {
                  message: 'Please Make a Selection'
              }
          }
       },// end dropdown
       startDate: {
           message: 'Please Make a Selection',
           validators: {
              notEmpty: {
                  message: 'Please Make a Selection'
              }
          }
       } // end dropdown

  } // end field
});// bootstrapValidator

Any ideas?