StringUtil.convertEncodedString returns System Error: Failed invoking <symbol>

I'm trying to use the Cryptography.Cipher class to decrypt a text I previously encrypted using the following configs:



This is the code I have atm:

hidden function convertStringToByteArray(plainText as String) as ByteArray  {
    var options = {
        :fromRepresentation => StringUtil.REPRESENTATION_STRING_PLAIN_TEXT,
        :toRepresentation => StringUtil.REPRESENTATION_BYTE_ARRAY,
        :encoding => StringUtil.CHAR_ENCODING_UTF8
    };                
    var result = StringUtil.convertEncodedString(plainText, options);
    
    return result;
}

hidden function convertByteArrayToString(byteArray as ByteArray ) as String {
    var options = {
        :fromRepresentation => StringUtil.REPRESENTATION_BYTE_ARRAY,
        :toRepresentation => StringUtil.REPRESENTATION_STRING_PLAIN_TEXT,
        :encoding => StringUtil.CHAR_ENCODING_UTF8
    };              
    var result = StringUtil.convertEncodedString(byteArray, options);
    
    return result;
}

function decryptKey(deviceId as String) as String {
    var key = convertStringToByteArray("ec41e0ef9dc3469bbb0cd7849e00a0e7");
    System.println(key.toString());
    var iv = convertStringToByteArray("0000000000000000");

    var cipher = new Toybox.Cryptography.Cipher({
        :algorithm => Toybox.Cryptography.CIPHER_AES256,
        :mode => Toybox.Cryptography.MODE_CBC,
        :key => key,
        :iv => iv
    });

    var encryptedString = convertStringToByteArray(deviceId);
    var encryptedBytes = cipher.decrypt(encryptedString);
    var result = convertByteArrayToString(encryptedBytes);
    return result;
}