Irregular alpha background

[grafo79]

Hello,

I created a common studio scene for a product, with no other objects around, but when I export a png files with alpha channel a piece of circular background appears that doesn't exist in the scene! 

Can anyone explain to me what it is and how can I solve?

Thanx a lot!

[mikeybabes]

I wonder if this is your environment being too small. change the background to the environment and take a look.