In present tense sentences that contain 'नहीं', है ( है/हैं/हो/हूँ) is optional. I am not entirely sure why that is but I've heard that it is probably because नहीं was derived from some variation of ना+है so tacking on another है is redundant.
I see. I did wonder if that was the case, but i thought that only applied to males - as the feminine form of words doesn't change for plurals. I assume for hai/hain it applies to both masculine and feminine. Thanks.