find_last_not_of not returning string::npos

[Toby]

It seems that string's find_last_not_of method isn't returning
string::npos:

void test() {
  string s = "  ";
  int i = s.find_last_not_of(' ');
  cout << "i = " << i << endl                       // prints -1
       << "string::npos = " << string::npos
       << endl;
}

On g++ (Ubuntu 4.3.2-1ubuntu12) 4.3. I get this output:

  i = -1
  string::npos = 4294967295

Is this a bug? Or am I mis-reading the documentation?