C++ Error: no matching function for call to 'toupper'

The following code raises the no matching function for call to 'toupper' error (C++ 11):

string s("some string");
if (s.begin() != s.end())
{
    auto it = s.begin();
    it=toupper(it);
}

This code is from C++ primer, fifth edition, chapter 3, about "introducing iterators".

Do you know why this error?