Array comparison and Loops In C - c

Question

I am taking this free online course so resources and help are fairly limited. They want a Vigenere cipher. My code is passing all testing and I thought it was done until I typed "ho1W aRE y0Ou? as the text and "heLLo" as they key. The execution is perfect except for the lowercase u which does not continue through the 'z' - 'a' loop and instead prints ' ' '. The code does does the 'z' to 'a' loop successfully in the 'W' in "how" and 'y' in "you". The key, "heLLo" is does repeat successfully and is not at the end of the strlen when it hits the 'u'. It is also not increasing by 1 on non-alphabetical characters. I'm not sure where to go from this point. Can anyone please offer some suggestions? Thanks!
#include <cs50.h>
#include <string.h>
#include <stdio.h>
#include <stdlib.h>
#include <ctype.h>
#include <math.h>
// Function to get string (text) from user
string Encrypt(void);
int main(int argc, string argv[])
{
// Exits with improper arguement count
if (argc != 2)
{
printf("You must enter one keyword when running the program.\n");
return 1;
}
// Sets key entered for command argument 1
string key = argv[1];
// Checks key to make sure a-z is entered. Exits if not.
for (int i = 0, word = strlen(key); i < word; i++)
{
if (isalpha(key[i]))
{
}
else
{
printf("Only letters are allowed for the key.\n");
return 1;
}
}
string text = Encrypt();
// Secret used to print out final message
char secret = 'a';
// K contorls array place in key
int k = 0;
// If text is entered and alpha: compares text[i] and key[k]
if (text != NULL)
{
for (int i = 0, len = strlen(text); i < len; i++)
{
if (isalpha(text[i]))
{
// Checks k poition to make sure it is within array
if (k == strlen(key))
{
k = 0;
}
// Converts key if text is lowercase
if (islower(text[i]))
{
secret = (((text[i] - 'a') + (key[k] - 'a')) % 26) + 'a';
printf("%c", tolower(secret));
}
// Converts key if text is uppercase
if (isupper(text[i]))
{
secret = (((text[i] - 'A') + (key[k] - 'A')) % 26) + 'A';
printf("%c", toupper(secret));
}
k++;
}
// If not alpha ignores loop and prints text char.
else
{
printf("%c", text[i]);
}
}
}
return 0;
}
string Encrypt(void)
{
printf("Enter your text.");
string text = GetString();
return text;
}

Answer 1

The problem is when it gets to the 'u' in the string, you are on the 'L' in your key. So when this code runs:
secret = (((text[i] - 'a') + (key[k] - 'a')) % 26) + 'a';
By substitution you have:
secret = ((('u' - 'a') + ('L' - 'a')) % 26) + 'a';
Hint: 'L' - 'a' = -21. 'u' - 'a' = 20. Hope you can figure the rest out from here, good luck.