C++ std::transform() and toupper() ..why does this fail?

I have 2 std::string. I just want to, given the input string:

1. capitalize every letter
2. assign the capitalized letter to the output string.

How come this works:

  std::string s="hello";
  std::string out;
  std::transform(s.begin(), s.end(), std::back_inserter(out), std::toupper);

but this doesn't (results in a program crash)?

  std::string s="hello";
  std::string out;
  std::transform(s.begin(), s.end(), out.begin(), std::toupper);

because this works (at least on the same string:

  std::string s="hello";
  std::string out;
  std::transform(s.begin(), s.end(), s.begin(), std::toupper);