Почему возникает эта ошибка?

• error: expected identifier before 'nullptr'
  
• error: expected ',' or '...' before 'nullptr'
  

#include <iostream>

using namespace std;

template <typename T>
class BinaryTree
{
private:
    struct Node
    {
        T data;
        Node *left, *right;

        Node(T data=T(), Node *left=nullptr, Node *right=nullptr)
        {
            this->data = data;
            this->left = left;
            this->right = right;
        }
    };

    Node root(T(), nullptr, nullptr);
};

int main()
{

}