operator < stopped working

All were working well until I decided to add a copy constructor.
Please see the program. I am getting

"c:\Program Files\Microsoft Visual Studio .NET 2003\Vc7\include
\functional(139) : error C2679: binary '<' : no operator found which
takes a right-hand operand of type 'const D' (or there is no
acceptable conversion)
"

Seems like I can't guarantee constant-ness anymore with the copy
constructor. How do I get around with this ?

// test-sets.cpp : Defines the entry point for the console
application.
//

#include "stdafx.h"
#include <iostream>
#include <set>
#include <map>

using namespace std;

class D
{

private:
   string a;

public:
   //Constructor
   D()
   {
   };
   //Destructor
   ~D()
   {
   };
   //Copy constructor
   D(D& d)
   {
   };
   std::string& getA()
   {
     return a;
   }
   bool operator < (D d) const
   {
     if(a.compare(d.getA()) <0){
       return true;
     }else {
       return false;
     }
   }
};

int main()
{
    typedef std::set<D> dsets;
    D *d = new D();
    dsets s;
    s.insert(*d);
  return 0;
}