Using std::mutex as member variable in a class

I have defined a class that has std::mutex my_mutex as its private member variable. But when I try to use it using lock_guard in a member function which is called from different threads, the compiler throws up a lots of errors. If I keep this mutex outside the class it works. The code is as follows

class ThreadClass
{
  std::mutex my_mutex;
  public: 
     void addToList(int max, int interval)
    {

      std::lock_guard<std::mutex> guard(my_mutex);
      for (int i = 0; i < max; i++) 
       {
            // Some operation
       }
    }
};


 int main()
 {
    std::thread ct(&ThreadClass::addToList,ThreadClass(),100,1);
    std::thread ct2(&ThreadClass::addToList,ThreadClass(),100,10);
    std::thread ct3(&ThreadClass::print,ThreadClass());

     ct.join();
     ct2.join();
     ct3.join();
  }

If the same my_mutex is kept out of the class then it works fine. So when the same variable is within the class and called within the member function acted on by a thread, does it treat like a static member?