ASCII Characters Messed Up in simple C Program

Question

I have written this simple program for an assignment, but when I input my text, the output gives me symbols instead of chars. any help would be appreciated. I do not know why my output appears that way, but the program seems to compile fine. Maybe it is working and I need to just do a base test with the math to see if it is functioning properly. In any event if anyone sees errors in this, feedback is much appreciated.

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

string Crypto(string, int); // rotation


int main(int argc, string argv[])
   {
    int k = 0;
    // error checing
    if (argc == 0 || argc == 1 || argc > 2)
   {
        // get mad
        printf("Enter 1 integer as an argument.  Stop messing around!\n Try Again: ");
        return 1;
   }
    else
   {
        //create command line arguments to be stored into k
        k = atoi(argv[1]);
        k = k * 1;
   }

    // Get text to be encrypted
    printf("Enter the text you want to encrypt: \n");
    string a = GetString();

    string b = Crypto(a, k);
    printf("%s\n", b);

    return 0;
 }

 //Now let's get cryptic
 string
 Crypto(string a, int k)
 {
    int c = 0;
    for (int i = 0, n = strlen(a); i < n; i++)
    {
        if(a[i] >= 65 && a[i] <= 90)
        {
            c = ((26 - (91 - a[i] + k) % 26));
            a[i] = c + 'A';
        }
        else
        {
            c = ((26 - (123 - a[i] + k % 26)));
            a[i] = c + 'a';
        }
     }
     return a;
  }

Answer 1

In function Crypto, you need to attach a \0 (null) character to signify the end of the string. Just before return a;, write a a[i+1] = '\0'; statement.