"Cannot convert from 'A' to 'B&'

I'm building an Entity-Component system using template metaprogramming. I keep getting either Cannot convert from [base type] to [type user requested]& or Cannot convert NullComponent to [type user requested]& errors:

class Entity {
public:
    Entity() = default;
    ~Entity() = default;

    template<typename C, typename... Args>
    void AddComponent(Args&&... args);

    template<typename C>
    C& GetComponent();

protected:
private:
    //...add/get helper methods here...

    unsigned int _id;
    std::vector<std::unique_ptr<IComponent>> _components;
};

template<typename C>
C& Entity::GetComponent() {
    for(auto c : _components) {
        if(std::is_base_of<a2de::IComponent&, C&>().value && std::is_same<decltype(c), C&>().value) {
            return *c; //<-- error here
        }
    }
    return NullComponent(); //<-- and here
}

It's 1 am and I can't figure this out...