6th shawaal fast falling in new month

I have been a bit delayed with my Shawwal fasts and am keeping the last one today, as I am following a pre-printed calendar with 30 Shawwal days.

However, on seeing the date on your website, I see that my 6th Fast now falls in the new month. I am fulfilling the fast, but is it still valid as 6 for Shawwal, or must I rather “keep” the other 5 as Nafl fast?