The news that Britney Spears will guest star on “How I Met Your Mother” apparently didn’t set well with Alicia Silverstone, who was to appear on the same episode as Britney.

As a result, Alicia Silverstone has bailed and “Scrubs” star Sarach Chalke (seen here in the Hanes Wedgie Free underwear ad) will replace Silverstone, playing a dermatologist. Britney Spears is set to play the dermatologist’s assistant.

This news comes from TVGuide.com, who reports that a HIMYM insider broke the news about Alicia Silverstone being out and Sarah Chalke taking her place. Alicia Silverstone was to appear in several episodes (which sparked debate that she may be the “mother” in “How I Met Your Mother”) — with Sarah Chalke stepping in, the role has been reduced to just one episode due to Chalke’s schedule.

Pink is the New Blog reported that Alicia Silverstone’s reps were worried that Britney Spears would overshadow Silverstone. True? Not sure, but there’s buzz that Alicia Silverstone may appear on “How I Met Your Mother” in another role.

The Britney Spears/Sarah Chalke episode of HIMYM will air near the end of April.