UndefinedBehaviorSanitiser Issue - CS50 Vigenere

I am working on Vigenere (CS50) and keep getting an "UndefinedBehaviorSanitiser SEGV on Unknown Address" when I run my program with any argument that passes the initial screening.

I have read about this issue but cannot find the solution. I cut my code down as much as I could and found the problem occurs even when I do this part. Where is the issue? Thank you so much.

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

//int shift(char c);

int main(int argc, string argv[])
{
    if (argc != 2)
        {
        printf("Usage: ./vigenere keyword");
        return 1;
        }
    else
        {
        for (int i = 0; i < strlen(argv[1]); i++)
            {
            if (!isalpha(argv[1]))
            {
                printf("Usage: ./vigenere keyword");
                return 1;
            }
            else
            {
                printf("All good!");
                return 1;
            }
         }
    }
}

Solution

The fix indeed is

if (!isalpha((unsigned char)argv[1][i]))

The isalpha function/macro takes only a single integer, which must have the value of a single character as unsigned char. But argv[1] is a pointer to multiple characters!

Now as an extra complication, the isalpha is often realized as a macro, and often coded so that a compiler does not produce any diagnostics for a wrong type of argument. This is unfortunate, but you just need to know these when you're programming in C.

The cast of char to unsigned char is required too - if not, then any extended characters (i.e. for example ä) will invoke undefined behaviour on platforms where char is signed - and they are on x86 processors - because the value will be negative, but isalpha would expect just a number that is either EOF or a non-negative number less than or equal to UCHAR_MAX.