In sort Linked List question, I am getting runtime error when I initialize fast=head and it is running fine when I initialize fast=head->next

Question

In sort Linked List question on leetcode, I am getting runtime error when I initialize fast=head and it is running fine when I initialize fast=head->next. What is the reason behind it? Can anyone explain.

class Solution {
public:
    ListNode* merge(ListNode* p, ListNode* q)
    {
        
        ListNode* dummy = new ListNode(0);
        ListNode* t = dummy;
        
        while(p && q)
        {
            if(p->val < q->val)
            {
                t->next = p;
                p = p->next;
            }
            else
            {
                t->next = q;
                q = q->next;
            }
            t = t->next;
        }
        t->next = (p) ? p : q;
        
        return dummy->next;
    }
    
    ListNode* sortList(ListNode* head) 
    {
        if(!head || !head->next)
            return head;
        
        ListNode *slow=head, *fast=head; //I am talking about this
        while(fast && fast->next)
        {
            slow = slow->next;
            fast = fast->next->next;
        }
        
        ListNode* head2 = slow->next;
        slow->next = NULL;
        
        ListNode* left = sortList(head);
        ListNode* right = sortList(head2);
        
        return merge(left, right);
    }
};