How to pass reference type to std::hash

I am creating a Template Cache library in C++-11 where I want to hash the keys. I want to use default std::hash for primitive/pre-defined types like int, std::string, etc. and user-defined hash functions for user-defined types. My code currently looks like this:

template<typename Key, typename Value>
class Cache
{
    typedef std::function<size_t(const Key &)> HASHFUNCTION;
    private:
        std::list< Node<Key, Value>* > m_keys;
        std::unordered_map<size_t, typename std::list< Node<Key, Value>* >::iterator> m_cache;
        size_t m_Capacity;
        HASHFUNCTION t_hash;

        size_t getHash(const Key& key) {
            if(t_hash == nullptr) {
                return std::hash<Key>(key);  //Error line
            }
            else
                return t_hash(key);
        }

    public:
        Cache(size_t size) : m_Capacity(size) {
            t_hash = nullptr;
        }

        Cache(size_t size, HASHFUNCTION hash) : m_Capacity(size), t_hash(hash) {}        void insert(const Key& key, const Value& value) {
            size_t hash = getHash(key);
            ...
        }
        bool get(const Key& key, Value& val) {
            size_t hash = getHash(key);
            ...
        }
};

My main function looks like this:

int main() {
    Cache<int, int> cache(3);
    cache.insert(1, 0);
    cache.insert(2, 0);
    int res;
    cache.get(2, &res);
}

On compiling the code above, I get the below error:

error: no matching function for call to ‘std::hash<int>::hash(const int&)’
             return std::hash<Key>(key);

Can anyone please help me out here and point out what am I missing or doing it incorrectly?