Ceasar Cypher: Decryption Key Index Outside the bounds

Exercise URL: https://www.codecademy.com/courses/learn-c-sharp/projects/csharp-caesar-cipher

When using a key to decrypt, i was told to use the modulo operator to “wrap around” the alphabet.

I understand how and why this works but i can’t say the same for the decryption process.

                char pos = encryptedMessage[i];
                int alphabetPos = Array.IndexOf(alphabet, decryptedMessage[i]);
                int newLetterPos = (alphabetPos - key) % 26;
                char decryptedChar = alphabet[newLetterPos];
                decryptedMessage[i] = decryptedChar;

This obviously gets me an index out of bounds.
I get why this doesn’t work the same way as with adding the key.
But i can’t get to a solution for the life of me.

I would appreciate a little pointer!