unhandled exception - access violation reading location

[smallmos]

i'm getting this error when i run my program:

Unhandled exception at 0x003f186f in Lab 8.exe: 0xC0000005: Access violation reading location 0x00000008.

here's the of code that i'm being told is where the error is (the line is in bold where the error is):

double DoubleList::deleteMostRecent()
{
	if(head == NULL)
	{
		return NULL;
	}
	else if(head->next == NULL)
	{
		double value = head->data;
		delete head;
		head = NULL;
		return value;
	}
	else
	{
		DoubleListNode *secondLastNode;
		secondLastNode = head;
		while(head->next->next != NULL)
		{
			secondLastNode = secondLastNode->next;
		}
		DoubleListNode *lastNode = secondLastNode->next;
		double value = lastNode->data;
		delete lastNode;
		secondLastNode->next=NULL;
		return value;
	}
}

Could someone explain this further to me and offer a possible fix?