Using mem_fun_ref on overloaded member function.

I am having trouble using mem_fun_ref on an overloaded member
function. Here's my code (using gcc 4.x):

#include <string>
#include <iostream>
#include <algorithm>
#include <vector>
#include <functional>

using namespace std;

int main()
{
    vector<string> vs;
    vs.push_back(string("One"));
    vs.push_back(string("Two"));

    for_each(vs.begin(),vs.end(), bind2nd (mem_fun_ref
( &string::push_back), 's' ) );
    cout << vs[0] << " and " << vs[1] << endl; //prints Ones and Twos as
expeced

    //ERROR: Following line does not compile
    for_each(vs.begin(),vs.end(), bind2nd(mem_fun_ref
(&string::append),"ome"));
    cout << vs[0] << " and " << vs[1] << endl ; //should output Onesome
and Twosome

    //This works, but not how I want to do it
    for (vector<string>::iterator it=vs.begin(); it!= vs.end(); it++)
        it->append("ome");
    cout << vs[0] << " and " << vs[1] << endl ; //outputs Onesome and
Twosome

}

gcc (4.0.1 on Max OS X) gives this :
 mem_fun_fun.cpp: In function =91int main()':
 2 mem_fun_fun.cpp|20| error: no matching function for call to
=91mem_fun_ref(<unknown type>)'

I am guessing the compiler has no idea which of the overloaded
string::append functions to use. Any ideas on solving this.