Whenever I use a rotobrush for my clip, the first frame of the clip turns all black, why?

Sometimes my roto does this randomly. Don't know why. I cannot reproduce the problem with a method. But every week randomly my roto will change the first frame to black. Nothing I can do about it. So now I save my work before I start to roto. Then I save before I freeze roto. Then save afetr I freeze. All i could do was roll back from a save when this happened.

Trying toggling the Alpha mode...may be it is set to Alpha mode by default. 

Refer the Screenshot below - 

if this does not work...then share the screeshot of the issue with us and also lets know the codec/format of the video your working with and After Effects verison.