Export Error CSS minification

Hi guys ,
I cant understant why ı am getting this error.
• CSS minification
Invalid background value at 2:14. Ignoring.

You have a error in one of your background values.

I would look at your custom css or 3rd party css.

For instance if I use:

.dig {
  background: "";

Error message “Invalid background value at 2:14. Ignoring”
So to fix I would use:

.dig {
  background: transparent;