Setting release to 'Unassigned' via external state

I am trying to set the 'Found In' attribute to 'Unassigned' using the RTC synchronization mechanism.

IWorkItem.UNASSIGNED_FOUND_IN_NAME is not recognized as a valid release name. 'null' value (which is what RTC return when 'Found In' is not assigned) seems to have no effect. That is, external connection shows OK synchronization status but different values for ItemValue (for instance some release) and External value (empty), which is really puzzling.

Is it at all possible to set found in to 'Unassigned'?

I expect similar issues with iterations.