after effects warning: roto brush strokes cannot be added until the current frame has finished rende

The following error message occurs;

"after effects warning: roto brush strokes cannot be added until the current frame has finished rendering"

It immediately and continually pops up when I try to use the brush. It is making the tool unuseable right now. Any ideas what might be causing this? I've only just updated it. 

I've cleared disk cache etc.