How to write a random uppercase char generator in C++?

My take on this would be

#include <iostream>

int main()
{
    for( int i = 0; i < 20; ++i) {
        std::cout << (char) ((random() % (int('Z') + 1)) + int('A'))  << '\n';
    }
}

This seems to be very straight forward. You generate a number, that is between everything lower or equal to Z and greater or equal to A. If you play with it, you will find that there are also other chars generated. Why is that?