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Isla Fisher serenaded by Katy Perry at 40th birthday

Isla Fisher serenaded by Katy Perry at 40th birthday

Katy Perry sang for Isla Fisher at her 40th birthday party in February.

The 'Confessions of a Shopaholic' star - who reached the milestone age on February 3 - has revealed that the 'Fireworks' singer was asked by her husband Sacha Baron Cohen to serenade the flame-haired beauty at a family bash and she decided to pretend she didn't know the words to all of her hits to avoid looking like a fan girl.

She shared: "I celebrated my 40th birthday with my friends, family - and Katy Perry. My husband [Sacha Baron Cohen] asked her to sing for me and she kindly obliged. I had to pretend that I didn't know the words to every single song so she wouldn't realise how big a fan I am!"

Meanwhile, Isla has accepted that ageing isn't "easy", but has learnt to love the new "old you" and she joked she'd like to be "tall enough" to be a supermodel.

She told YOU magazine: "Ageing isn't easy. But for me, I think it's just a matter of reframing how you think about the new 'old' you. One of my friends recently described me as the 'oldest young person I know' - which is kind because I'm now middle-aged. All I've got to achieve is learning to think before I speak and getting tall enough to be a supermodel."

Isla - who has Olive, eight, Elula, five and 14-month-old son Montgomery with her 'Zoolander' actor husband - said her biggest aim in life is to be "considered cool" by her kids.

And she's also like to work with the Coen brothers.

She said: "My biggest aim is to be considered cool by my kids. I'd also like to write more children's books and, on the acting side, work with Coen brothers, David Fincher, Spike Jonze and Adam McKay - preferably, not as their assistant."