My code complies well but encrypts the text in the wrong way.Could someone help me find the problem?

any help will be appreciated

here is my code:

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

int main (int argc, string argv[])
{
if (argc != 2)
{
     printf("Please enter a valid key\n");
    return 1;
}

string key = argv[1];

for (int i=0 ,n=strlen(key);i<n; i++)
{
    if(!isalpha(key[i]))
    {
        return 1;
    }
}

printf("input:");

string text = GetString();
int k = 0;

for(int i = 0,n = strlen(text),j = k % strlen(key); i < n ; i++,j++)
{
   
    
    if(isupper(text[i]))
    {
        char c = ((text[i]-65 + key[j])%26) + 65;
        printf("%c",c);
    }
    
    else if (islower(text[i]))
    {
        char c = ((text[i]-97 + key[j])%26) + 97;
        printf("%c",c);
    }
    
    else
    {
        printf("%c",text[i]);
    }
    
}
 printf("\n");
 return 0;
}