Uncaught TypeError: Cannot read property 'querySelector' of undefined in bindTaskEvents

//Problem: User interaction doesn't provide desired results.
//Solution: Add interactivity so the user can manage daily tasks.

var taskInput = document.getElementById("new-task"); //new-task;
var addButton = document.getElementsByTagName("button")[0];//first button
var incompleteTasksHolder = document.getElementById("incomplete-tasks"); //ul "#incomplete-tasks"
var completedTasksHolder = document.getElementById("completed-tasks"); //ul "#completed-tasks"

//Add a new task

var addTask = function() {
  console.log("Add Task...");
  //when the button is pressed
  //create a new list item with the text from the new-task
    //input checkbox
    //label
    //input (text)
    //button .edit
    //button .delete
    //Each element will need to be modified and appended


}


//edit an existing task

var editTask = function() {
    console.log("edit Task...");
    // when the edit button is pressed
    //if class of the parent is .editMode
      //switch from .editMode 
        //label text become the input's value
      //else
        //switch to .editMode
        //input value becomes the label's text

       //toggle .editMode on the parent


}


//delete a task
var deleteTask = function() {
    console.log("delete Task...");
    //when the delete button is pressed
    //remove the parent list item from the ul
}





//mark a task as complete
var taskCompleted = function() {
    console.log("complete Task...");
  //when the checkbox is checked 
    //append the task list item to the completed-tasks
}


//Mark a task as incomplete
var taskIncomplete = function() {
    console.log("incomplete Task...");
  //when the bheckbox is checked
    //append the task list item to the incomplete-tasks
}


var bindTaskEvents = function(taskListItem, checkBoxEventHandler) {
  console.log("Bind list item events");
   //select it's children
  var checkBox = taskListItem.querySelector('input[type="checkbox"]');
  var editButton = taskListItem.querySelector("button.edit");
  var deleteButton = taskListItem.querySelector("button.delete");
  //bind editTask to edit button
  editButton.onclick = editTask;

  //bind deleteTask to delete button
  deleteButton.onclick = deleteTask;

  //bind checkBoxEventHandler to checkbox
  checkBox.onchange = checkBoxEventHandler;
}

//set the click handler to the addTask function
addButton.onclick = addTask;

//cycle over incompleteTasksHolder ul list items
for(var i = 0; i < incompleteTasksHolder.children.length; i++) {

  //bind events to list items children (tasksIncompleted)
  bindTaskEvents(incompleteTasksHolder.children[i], taskCompleted);
}



//cycle over completeTasksHolder ul list items

for(var i = 0; i < incompleteTasksHolder.children.length; i++) {

  //bind events to list items children (taskCompleted)
  bindTaskEvents(completedTasksHolder.children[i], taskIncomplete);
}