No submodule mapping found in.gitmodules for path*** when trying to push to heroku

Hello.

When I try to upload the rails application to heroku, I get the following error:

No submodule mapping found in.gitmodules for path'app/assets/stylesheets/includes/sanitizes.css'

Remove the above path from cache management and remove the submodule in .git/config I tried, but it didn't go away... sanitize.css is a folder with sanitize.scss and surrounding files in it.

What should I do to cure it?Please help me.
Thank you for your cooperation.