posts - call to jquery ajax failing with 500 error when passing extra data field

I have the following jQuery for doing an Ajax call and it works fine:

jQuery(document).ready( function($){
$( "#mystatus" ).change(runMyStatus);

 function runMyStatus() {
  var mystatus = this.value;
  var data = {
        'action': 'bikeride_mystatus_function',
        'mystatus': mystatus,
        // 'rideID' : 123,
        'nonce': frontEndAjax.nonce
    };

   var ajaxRequest = $.ajax({
       url: frontEndAjax.ajaxurl,
       type: "post",
       data: data
   });
   ajaxRequest.done(function(response) {
     alert('Success');
   });
   ajaxRequest.fail(function(jqXHR, textStatus, exception) {
     alert('Error.\n' + jqXHR.status + ' ::' + textStatus + ' ::' + exception);
   });
 } // end runMyStatus
// end document ready
});

If I uncomment the line with rideID then it fails with a "500" status (I tried putting 123 in quotes but it made no difference). If I change the type from "post" to "get" then it works. This error occurs even with no plugins enabled and using the twentyseventeen theme. Any idea what is causing the error?