Uncaught TypeError: Cannot read property 'querySelector' of undefined.. What have I missed?

// Problem: User interaction doesn't work
// 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-task

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

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

var taskCompleted = function() {
    console.log("Complete task...");
}

// Mark a task as incomplete
var taskIncomplete = function() {
    console.log("Incomplete task...");
}

var bindTaskEvents = function(taskListItem, checkBoxEventHandler) {
    console.log("Bind li events");
    // select li's children
    var checkbox = taskListItem.querySelector("input [type=checkbox]"); //edit needed a space between input and [] //////
    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 checkBoxEventHandler to checkbox
    checkbox.onchange = checkBoxEventHandler;
}

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

// Cycle over incompleteTasksHiolder ul list items
for (var i = 0; incompleteTasksHolder.children.length; i++) {
        // bind events to li' children (taskCompleted)
    bindTaskEvents(incompleteTasksHolder.children[i], taskCompleted);
}     

// Cycle over completedTasksHiolder ul list items

for (var i = 0; completedTasksHolder.children.length; i++) {
        // bind events to li' children (taskCompleted)
    bindTaskEvents(completedTasksHolder.children[i], taskIncomplete);
}