Failing one test case with check50 on pset2 - encrypts "say" to "szz" instead of "rby"

Question

encrypts "world, say hello!" as "xoqmd, rby gflkp!" using "baz" as keyword
#   Checks  
0    Running...  clang -o vigenere vigenere.c -lcs50 -lm. ✔
1    Expecting an exit code of  0. ✔
2    Running...  ./vigenere baz. ✔
3    Takes in standard input of  world, say hello!. ✔
4    Expecting the following on standard out —  xoqmd, rby gflkp!\n  ... but received the following on standard out instead —  xoqmd, szz gflkp!

My vigenere.c fails on only this test case. The rest pass properly. I'm no sure if posting my source code here would violate the honor code, hence posting just the error instead.

I'm supposed to get rby instead of szz for the plaintext - say, otherwise the rest of the ciphertext is also right as you can see.

Any ideas as to what I might be doing wrong?

UPDATE:

I'm trying to implement @superindesu's suggestions but the key that I am trying to use is generating funny.

I am using isalpha(char_variable) and ispunct(char_variable) but it's misreading a portion of the input and generating a wrong key. For debugging purposes, I've used - to indicate output from ispunct() and # to indicate output from an else block which catches anything other than alphabets or punctuations:

jharvard@appliance (~/Dropbox/pset2): ./vigenere-2 baz
world, say hello!
bazba-#zba#-#zba-# - key

Any ideas on how to fix this?

UPDATE 2: (Code)

string plaintext = GetString();

int keylen = strlen(key);
int ptlen = strlen(plaintext);

int j=0;
for(int i=keylen;i <= ptlen;i++){
    if(isalpha(plaintext[i])){
        key[i]=bigkey[j];
        j++;
    }
    else if(ispunct(plaintext[i])){
        key[i]='-';
    }
    else{
        key[i]='#';
    }
    key[i+1]='\0';
}

printf("%s - key\n",key);

// bigkey is a string that has the key repeated to the length of the plaintext.
// key is the key provided at the command-line by the user 

Answer 1

2 points:

1- The key mustn't be applied until the character is alphabetical, so (- means no encoding)

You are doing this:
String: w o r l d ,   s a y   h e l l o !
Key:    b a z b a z b a z b a z b a z b a

While the right approach is:
String: w o r l d ,   s a y   h e l l o !
Key:    b a z b a - - z b a - z b a z b -

2- You have forgotten to include a return code at the end of the encoded line "\n"