error: operator=() does not handle self-assignment properly

I am running clang-tidy on the code below:

BinaryTree& operator=(const BinaryTree &bt){ 
    if(this == &bt){ 
        return *this;
    } 
    if(_root != nullptr) { 
        delete _root; 
    } 
    _root = new node(nullptr, bt._root->_data); 
    copy_con(_root, bt._root); 
    return *this; 
}

It gives the following error:

Error: operator=() does not handle self-assignment properly.

This is strange, since my code does handle self-assignment in the first line!

How can I fix this?