各位大佬们，小弟有个字典树里指针调用的问题望赐教！

关于字典树的一个疑问，为什么总是在p->child[pos]=NULL上报错读取访问权限冲突？

void insert_I(TrieTreeNode* p, const string& word)
    {
        for (int i = 0; i < word.length(); ++i)
        {
            int pos = word[i] - 'a';
            if (p->child[pos] == NULL)
            {
                p->child[pos] = new TrieTreeNode();
                p->child[pos]->ch = word[i];
                p->child[pos]->parent = p;
            }
            p->child[pos]->PrefixCount++;
            p = p->child[pos];
        }
        p->WordCount++;
    }