edx-platform - Error "...could not be found with..." when compiling assets

Yesterday, I got this error when trying to compile assets of our OpenEdX instance's Studio:

with this command:

~/edx-platform: paver update_assets cms --settings=aws

First, there was no file locates at themes/ourtheme/images/bg.png . Then what?

I took a tour into the template directory and check the sass file at  /edx/app/edxapp/themes/unschool/static/sass/_ourthem.scss

There were one variable which was defined with wrong url or didn't been used anymore in the template:

$homepage-bg-image: url('../themes/ourtheme/images/bg.png');

So, I just comment out that var and run the compile assets command again. Everything's ok now.