Task is to separate visits to Users (visitors who has user_id) and Not users (who has not).
How to reproduce:

1. Make visit without setting userID
2. Make another visit from another browser with _paq.push(["setUserId", 'userid@mydomain.org'])
3. Go to Piwik, switch to the Visitors - Visitor Log tab. There are 2 visits - without userID (visit1) and with it (visit2).
4. Create new Segment: name - Not Users, Metric: User ID, Match: Is, Value: Empty string
5. Create new Segment: name - Users, Metric: User ID, Match: Is Not, Value: Empty string

Expected results:

• Segment Not Users: only visit1
• Segment Users: only visit2

Actual results:

• Segment Not Users: Empty list
• Segment Users: visit1, visit2

Please see #6954