string validation isalpha always false?

So I've got a function for validating a string, and I've looked at the same issue I am getting elsewhere but fixes don't seem to work for my code any help would be appreciated :)

    bool validate(string s)
{
    for (int i = 0; i < s.size(); i++)
    {
        if (isalpha(s[i]) != 0)
        {
            return false;
        }
        if (isspace(s[i]))
        {
            return false;
        }
        if (ispunct(s[i]))
        {
            return false;
        }
    }
    return true;
}

The problem I am getting is it always returns false no matter what I've tried with and without != 0 but this doesn't change anything, generally the string I'm putting in is "blue" for anyone that is curious to how I am testing this.