Deep copying Queue using nodes runtime error

Question

I'm trying to perform a deep copy for my Queue (FIFO) using nodes and I'm getting an error and Don't know what's causing it. All the methods work fine when I don't copy anything but as soon as I try to copy I get a "MyQueue.exe" stopped working. This happens when I don't even use the methods. It also happens even if I comment out the whole do-while loop. Anyone know what's causing this?

MyQueue::MyQueue (void)
{
    head = NULL;
    queueSize = 0;
}

MyQueue::MyQueue (const MyQueue & myq)
{
    copyQueue (myq);

}

MyQueue & MyQueue::operator = (const MyQueue & myq)
{

    copyQueue (myq);
    return *this;

}

void MyQueue::copyQueue (const MyQueue & myq)
{
    head = NULL;
    queueSize = 0;
    Node* temp = myq.head->next;
    do
    {
        enqueue(myq.head->data);
        temp = temp->next;
    }
    while ( temp->next != NULL );

}

MyQueue::~MyQueue (void)
{

}

void MyQueue::enqueue (const int n)
{
    Node* temp = tail;
    tail = new Node (n);
    queueSize++;
    if ( empty() )
    {
        head = tail;

    }
    else
    {

        temp->next = tail;

    }

}

int MyQueue::dequeue (void)
{
    if ( !empty() )
    {
        int result = head->data;
        Node* temporary = head;
        head = (temporary->data, temporary->next);
        queueSize--;
        return result;

    }
    else
    {
        std::cout << "No Items to remove" << std::endl;
        return NULL;
    }

}

int MyQueue::peek (void)
{
    return head->data;
}



bool MyQueue::empty (void)
{
    if ( head == NULL || queueSize == 0)
    {
        return true;
    }

    return false;
}

This is how my node is copied

Node & Node::operator = (const Node* & nd)
        {
            data = nd->data;
            next = nd->next;
        }

Answer 1

MyQueue q1;
MyQueue q2 = q1; // Crash.

This will crash, because the head is NULL, and you dereference it in the copyQueue() method.

Node* temp = myq.head->next;