istream problem

I can't seem to fix this bug -- it uses and istream.  The first call works fine but the 2nd call does not: (Thanks in advance for your assistance!)

// UMBox2.cpp : Defines the entry point for the console application.
//

#include "stdafx.h"
#include <windows.h>
#include <string>
#include <sstream>
using namespace std;


//prototypes:
string cistream_to_string(istream MyStream);
long FullUMBox(HWND hwnd,istream MsgStm,const string Title,UINT style);


int main()
{
     long ret;
     stringstream S;
     S << "line one" << endl << "line two" << endl << 123.456;
     //this displays the correct result:
     ret=FullUMBox(NULL,S,"TEST1",MB_OK);
     //this doesn't work:
     ret=FullUMBox(NULL,S,"TEST2",MB_OK);
     return 0;
}


string cistream_to_string(istream S)
{
     string text;
     char ch;
     while (true) //infinate loop
     {
          ch = S.get();
          if (S.eof()) break;
          text += ch;
     }
     return text;
}


long FullUMBox(HWND hwnd,istream MsgStm,const string Title,UINT style)
{
  string text = cistream_to_string(MsgStm);
  return MessageBox(hwnd,text.c_str(),Title.c_str(),style);
}