caesar_segmentation_fault

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

bool count_argv(int argc);

bool only_digits(string s);

int string_to_int(string s);

char rotate(char c, int h);

int main(int argc, string argv[])
{
    bool n1 = count_argv(argc);

    bool n2 = only_digits(argv[1]);

    const int key = string_to_int(argv[1]);

    if (n1 == 0 && n2 == 0)
    {
        string text = get_string("plaintext: ");

        printf("ciphertext: ");

        for (int i = 0, n = strlen(text); i < n; i++)
        {
            printf("%c", rotate(text[i], key));
        }
        printf("\n");
    }
}

bool count_argv(int argc)
{
    if (argc != 2)
    {
        printf("Usage: ./caeser key\n");
        return 1;
    }
    else
    {
        return 0;
    }
}

bool only_digits(string s)
{
    int j = 0;
    do
    {
        j++;
    }
    while (isdigit(s[j]));
    int t = strlen(s);
    if (j == t)
    {
        return 0;
    }
    else
    {
        printf("Usage: ./caesar key\n");
        return 1;
    }
}

int string_to_int(string s)
{
    int m;
    m = atoi(s);
    return m;
}

char rotate(char c, int h)
{
    int cc;
    if(isalpha(c))
    {
        if(isupper(c))
        {
            cc = c - 'A';
            int k = (cc + h) % 26;
            c += k;
        }
        else
        {
            cc = c - 'a';
            int k = (cc + h) % 26;
            c += k;
        }
        return c;
    }
    else
    {
        return c;
    }
}