ncover and nunit

I always get an empty Coverage.xml and no errors in the verbose log. How can I get it to work?

set path=%CD%;%path%
NCover.Console //a "MRC_AO3API.dll" //w "..\..\bin\Debug" "..\nunit\bin\nunit-console.exe" "..\..\bin\Debug\MRC_AO3API_UnitTester.dll" /fixture=MRC.Tiramisu.Announcement.AnnouncementBPCTest.GetAnnouncementListTest //s