Tori Spelling won’t be letting it ride in Las Vegas this weekend … revealing she’s sick and has to cancel an appearance with her ex in Sin City.

The actress announced the news via Instagram Friday … telling her fans she reluctantly couldn’t host the ’90s After Dark Party on Sunday with Brian Austin Green because she’s dealing with a major illness.

Spelling tells her fans it hurts her to cancel her appearance … but she needs to prioritize her health this holiday season — and, she thanks fans for their love and understanding.

Of course, Tori and Brian famously played a very popular couple on “Beverly Hills, 90210” back in the ’90s … and their courtship even leaked over into real life.

TS and BAG dated in the early 1990s … and, despite the breakup, the two have maintained a good relationship over the years — with Brian even joining Tori for an episode of her podcast last year.