Incorrectly seeding Mersenne Twister via constructor

What is wrong with my constructor? Every time I call a function that is supposed to generate random numbers, it generates the same numbers. Each call instantiates one of these objects below. I thought I was seeding m_gen randomly with the output of m_rd's operator() call.

Could I pass in the result of m_rd() to the constructor? What would the signature be? Shuffler(std::random device& rd)? But then that would be more difficult for the user.

shuffler.h

#include <random>

class Shuffler
{
private:
    std::random_device m_rd;
    std::mt19937 m_gen;
public:

    //! The default constructor. 
    Shuffler();

};

shuffler.cpp

#include "shuffler.h"

Shuffler::Shuffler() : m_gen(m_rd())
{
}