Using template for return value. how to handle void return?

I have structure for storing callback function like this:

template<class T>
struct CommandGlobal : CommandBase
{
    typedef boost::function<T ()> Command;
    Command comm;

    virtual T Execute() const
    {
        if(comm)
            return comm();
        return NULL;
    }
};

Seems like it should work fine except when T is void because the Execute function wants to return a value..

What is the best solution to this problem?

Thanks!