Error in CSS file: Module not found

[ghost]

Actual behaviour

Karma throws errors about referenced images in CSS files through "url". E.g.:
background-image: url("my-image.png")
These CSS files are referenced through the "styleUrl" metadata property of the components.
The references are correct, so I would expect it not to throw errors.

Environment Details

• Karma version (output of karma --version):
  Karma version 1.7.1

Notes

I'm desperate to solve this issue. I have tried many, many things. I just karma to exclude or ignore these references but I can't find a way. Thank you.