Why does my random string generator produce too short strings?

I have written a simple function that returns a random string.

std::string cache::generateCacheName()
{
    static const char pool[] = "0123456789abcdefghijklmnopqrstuvxyzABCDEFGHIJKLMNOPQRSTUVWXYZ";
    std::string r;
    std::random_device rd;
    std::mt19937 eng(rd());
    std::uniform_int_distribution<> distr(0, sizeof(pool) - 1);
    for (int i = 0; i < 48; i++)
        r += pool[distr(eng)];
    return r;
}

However, sometimes it returns a string randomly shorter than 48 characters.

I already added the - 1 behind the sizeof(pool) trying to avoid adding the null-terminator from pool, but that didn't change anything.

Where did I go wrong?