unique_ptr constructor with rvalue deleter returns null?

std::default_delete<int> d;
    std::unique_ptr<int> u1;
    std::unique_ptr<int> u2 (nullptr);
    std::unique_ptr<int> u3 (new int);
    std::unique_ptr<int> u4 (new int, d);
    std::unique_ptr<int> u5 (new int, std::default_delete<int>());
    std::unique_ptr<int> u6 (std::move(u5));
    std::unique_ptr<int> u7 (std::move(u6));
    std::unique_ptr<int> u8 (std::auto_ptr<int>(new int));

    std::cout << "u1: " << (u1?"not null":"null") << '\n';
    std::cout << "u2: " << (u2?"not null":"null") << '\n';
    std::cout << "u3: " << (u3?"not null":"null") << '\n';
    std::cout << "u4: " << (u4?"not null":"null") << '\n';
    std::cout << "u5: " << (u5?"not null":"null") << '\n';
    std::cout << "u6: " << (u6?"not null":"null") << '\n';
    std::cout << "u7: " << (u7?"not null":"null") << '\n';
    std::cout << "u8: " << (u8?"not null":"null") << '\n';

Can somebody explain why null is printed for u5 and u6, but not null for u7? Thanks in advance.