How can I stop the loop through the keyword when there is a nonalpha character in Vignere?

Question

The code doesn't encrypt the nonalpha characters, so that is good, but it continues the count through the keyword rather than pausing it. So I encounter this problem:

Expected: ./vigenere baz plaintext: hello, world! ciphertext: iekmo, vprke!

Actual: ./vigenere baz plaintext: hello, world! ciphertext: iekmo, wnslc!

I am brand new to computer science, so I'm sure I'm missing something obvious. Here is my code:

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

int shift(char c);

string keyword; 

int key;

int main(int argc, string argv[])
{
    // check for single command-line argument
    if (argc == 2) 
    {
        // isolate key
        keyword = argv[1];
        // iterate through key to verify that each character is a digit
        int keys[strlen(keyword)];
        for (int i = 0, n = strlen(keyword); i < n; i++)
        {
            if (! isalpha(keyword[i]))        
            {
                printf("Usage: %s keyword\n", argv[0]);
                return 1;
            }
            else
            {
                key = shift(keyword[i]);
                keys[i] = key;
            }
        } 
        // get plaintext from user
        string s = get_string("plaintext: ");
        printf("ciphertext: ");
        // iterate over each character of plaintext
        for (int i = 0, n = strlen(s); i < n; i++)
        {
            // if uppercase, rotate, preserve case, and print out new character
            if (isupper(s[i]))
            {

                printf("%c", (((s[i] - 65) + keys[i % strlen(keyword)]) % 26) + 65);
            }
            else
                // if lowercase, rotate, preserve case, and print out new character
                if (islower(s[i]))
                {
                    printf("%c", (((s[i] - 97) + keys[i % strlen(keyword)]) % 26) + 97);
                }
            // if character, print out as is
                else
                {
                    printf("%c", s[i]);
                }
        }
        printf("\n");
        return 0;
    }    
    else
    {
        printf("Usage: %s keyword\n", argv[0]);
        return 1;
    }
}
int shift(char c)
{
    int k;
    //if uppercase
    if (isupper(c))
    {
        k = (((int) c) - 65);
    }
    //if lowercase
    else
    {
        k = (((int) c) - 97);
    }
    return k;
}