Potty Trained but not on carpet! Any Ideas?

[kewlgimp07]

My little dog Buster is a Yorkie Shih-Tzu and he's been doing really good at the potty training thing recently, but our upstairs is all hardwood floors and our downstairs is all carpet, why does he immediately or soon after he's on the carpet, go potty every single time? Even if he's just went 5 minutes ago outside? Maybe cuz he's not used to the carpet? He pottied on my sisters carpet at her house too, right after he'd just pottied outside. Any suggestions would be great, thanks.