Using unique_ptr in a pair

My code:

template <typename T>//, T value_if_empty>
class AtomicQueue {
    std::mutex m;
    std::queue<T> q;
    T value_if_empty;

public:
    AtomicQueue(const T& emptyval) : value_if_empty(std::move(emptyval)) {};
    void push(const T& t)
    {
        m.lock();
        q.push(std::move(t));
        m.unlock();
    }
    T pop()
    {
        T a = std::move(value_if_empty);
        m.lock();
        if (!q.empty())
        {
            a = std::move(q.front());
            q.pop();
        }
        m.unlock();
        return a;
    }
};

AtomicQueue<int> A(0);
AtomicQueue<std::unique_ptr<Class1>> B(nullptr);
AtomicQueue < std::pair<int, std::unique_ptr<Class1>>> C({ 0,nullptr });

This code compiles if I declare just A and B but fails when C is also declared. The error is 'std::pair<int,std::unique_ptr<Class1,std::default_delete<Class1>>>::pair(const std::pair<int,std::unique_ptr<Class1,std::default_delete<Class1>>> &)': attempting to reference a deleted function

The problem seems to be that std::pair<int, std::unique_ptr<Class1>> does not have a copy constructor. But the same is true for std::unique_ptr<Class1>. So why does the declaration for B compile but C does not? Also, where exactly is the copy constructor being used? Sorry if the answer is trivial. I am new to using unique_ptr and move constructors.

Side Question: Is there a cleaner or better way to use value_if_empty? I do not want anything other than nullptr being passed when unique_ptr is being used.