Searchbuilder contactsubtype ≠ 'value' fails

When trying to set up a search selection through the searchbuilder I want to select all contacts that do NOT have a specific contact subtype. But whether I select = or ≠ the result stays the same (the one for =)

I simulated this on the demoserver
1) Make sure there is a contact with contactsubtype 'volunteer'
2) Use searchbuilder and select all contacts with contact subtype ≠ volunteer.

Expected behaviour:

• get a (long) list of all contacts with contact subtype ≠ volunteer
  Real behaviour
• get a short list with the contacts with contact subtype = volunteer
  (which is exactly the opposite of the expected behaviour)

I assume this is only a small bug to fix. Maybe someone can fix this easily or point me to the file and section in which I can try to fix this problem myself.