Should captcha or verification codes be case sensitive?

As I have come across many verification codes, the most annoying thing in them is when you write in all letters in the code in lower case and the code is rejected saying its not valid. Some codes are accepted and some are not. And the user has no idea if they are case sensitive or not before actually submitting it and validating it. Given the curvy, hardly recognisable characters on the codes, not making the users aware of its case sensitivity could annoy them more and bring down the UX.

So what would be the ideal way to deal with this ??