bindTaskEvents gives "Uncaught TypeError: Cannot read property 'querySelector' of undefined "

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

//New Task List Item
var createNewTaskElement = function(taskString){
    var listItem = document.createElement("li");
    //input (checkbox)
    var checkBox = document.createElement("input"); //checkbox
    //label
    var label = document.createElement("label");
    //input(text)
    var editInput = document.createElement("input");
    //button.edit
    var editButton = document.createElement("button");
    //button.delete
    var deleteButton = document.createElement("button");
    //Each element needs modifying

    //Each element needs appending
    listItem.appendChild(checkBox); 
    listItem.appendChild(label); 
    listItem.appendChild(editInput); 
    listItem.appendChild(editButton); 
    listItem.appendChild(deleteButton); 

    return listItem
}


// Add a new task
var addTask = function() {
    console.log("Add task...")

    var listItem = createNewTaskElement("Some new task");
    //Append listItem to incompleteTaskHolder
    incompleteTaskHolder.appendChild(listItem);
    bindTaskEvents(listItem, taskCompleted);
}

// Edit an existing task
var editTask = function() {
    console.log("Edit task...");
}

// Delete a task
var deleteTask = function() {
    console.log("Delete task...");
}


// Mark a task as complete
var taskCompleted = function() {
    console.log("Task complete...");
    var listItem = this.parentNode;
    completedTaskHolder.appendChild(listItem);
    bindTaskEvents(listItem.taskIncomplete);
}

// Mark a task as incomplete
var taskIncomplete = function() {
    console.log("Task incomplete...");
    var listItem = this.parentNode;
    incompleteTaskHolder.appendChild(listItem);
    bindTaskEvents(listItem,taskCompleted);

}

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


//set the click handlet to the addTask function

addButton.onclick = addTask

//Cycle over incompleteTaskHolder ul list items
for(var i = 0; i < incompleteTaskHolder.children.length; i++) {
    bindTaskEvents(incompleteTaskHolder.children[i], taskCompleted);
}
//Cycle over completeTaskHolder ul list items
for(var i = 0; i < completedTaskHolder.children.length; i++) {
    bindTaskEvents(completedTaskHolder.children[i], taskIncomplete);
}