I used this simple check for greater than 0.

    int key;
    if( (argc<2) || (argc>2))
    {
        printf("Usage: ./caesar key\n");
        return 1;
    }
    else if (atoi(argv[1])==0)
    {
        printf("Usage: ./caesar key\n");
        return 1;
    }
    else
    {
        key = atoi(argv[1]);
    }

    string plaintext = get_string("plaintext: ");

I am still getting the same error. My program exits with error code 1 before prompting for plaintext. Idk, why the CS50 check still tells me timed out.