Segment excluding "Unknown" Providers possible?


I would like to build a segment that excludes (via “is not” or “does not contain”) all visitors with Providers indicated as “Unknown”. As by nature “unknown” providers cannot be specified via a provider URL, I was wondering if there is a workaround to build a segment (via drag-and-drop condition). To enter the value “unknown” doesn’t lead to the desired exclusion as the required value apparently needs to be a URL.

Thank you!