Removing `else` condition suddenly makes the code work, what gives? [closed]

This code doesn't do as intended - it should print out all of plaintext. I'm guessing there's something at work under the hood, but that logic escapes me. If you remove the else condition with underlying statement, it suddenly works.

#include <cs50.h>
#include <ctype.h>
#include <stdio.h>


int main() {

    string plaintext = get_string("plaintext: ");
    
    int i;
    for(i = 0; plaintext[i] != '\0'; i++)
        if(isalpha(plaintext[i] != 0))
            printf("%c", plaintext[i]);
        //I intend to do stuff with alphabetic characters, but that code isn't relevant, so it's not included
        else
            printf("%c", 'a');
}

I'll be honest, this looks like magic to me. Why would adding an else condition affect whether the condition for the original if statement was met or not (if that's the case)?

Is it somehow because of using string in cs50.h?