Post WIP to G/L in Fixed price project

Hi,

I have a question on Posting WIP to GL.
I noticed that whenever i have a billable line and budget lines and I want to post WIP to G/L. the system asks me to remove the gen prod posting group from the revenue account.
this is happeneing only when i post wip to G/L for bothe line type budget and billable together.
When i have only billable lines im not getting this message.

Please advise