Won't Select Rows With Null Field Values

I cannot in the life of me understand what goes wrong here... In a webapplication (C#.NET) I traced an inability to retrieve existing records to SQL Server, where I cannot do the same either. The problem is that in the parameterized query, some fields can be null, and thus when the corresponding fields in the database record are null also, they should be selected. But this won't happen for some reason.

I wrote a test SQL statement that gives me the same bogus:

DECLARE @institution int, @collection int, @serialnr int, @subnr nvarchar(50)SET @institution = 1 SET @collection = 1SET @serialnr = 240 SET @subnr = NULLSELECT ID, Institution, Collection, SerialNumber, SubNumber, AccessionYear, Sagsnummer, DanekraeNr, TaxonIdentified, Stratigraphy, TypeStatus, PlacementRoom, PlacementCabinet, PlacementDrawer, UnitNotesFROM SpecimensWHERE (Institution = @institution) AND (Collection = @collection) AND (SerialNumber = @serialnr) AND (SubNumber = @subnr)
Now there is at least one row with corresponding fields values (1, 1, 240, null), but it won't be selected! What is wrong!?