After Effect with a line around the mask

Hello, 

I have a bug with after effect 

I want to animate a mask in a photo

when i animate this, i have all around the mask a line apears

The same with vidéo or solid

1- i create a solid

2- i create a mask and animate this 

3- i duplicate the layer and i put the mask on "soustraction"

4-then i look a line , (this is black beacause is the back, )

I need your help, i do everything,  delete the app, delete the cache

Can u help me ? 

My computer, is very good for do this, 64GB, GTX 3070, Intel 9