Getting an error on line 19 for send method

var xhr = new XMLHttpRequest();
xhr.onreadystatechange = function() {
  if(xhr.readyState === 4) {
    var employees = JSON.parse(xhr.responseText);
    var statusHTML = '<ul class="bulleted">';
    for(var i = 0; i < employees.length; i += 1) {
      if(employees[i].inoffice === true) {
      statusHTML += '<li class="in">';  
   } else {
         statusHTML += '<li class="out">';
   }
         statusHTML += employees[i].name;
         statusHTML += '</li>';
  }
         statusHTML += "</ul>";
         document.getElementById('employeeList').innerHTML = statusHTML;
};
xhr.open("GET", 'data/employees.json');
xhr.send(null);