Comparing null values in Caché

I have this query:

SELECT  *
FROM    Ens.MessageHeader head ,EnsLib_HL7.Message as body
WHERE   head.MessageBodyId = body.ID
AND     body.Name IN ('ADT^A34','ADT^A40')
AND     MessageBodyClassName = 'EnsLib.HL7.Message'

Which gives results like this:

I want to only get the records where BusinessProcessId is null so I add this line:

AND     BusinessProcessId IS NULL

But now I get no results.

I've tried a few variants but nothing seems to do what I expect. What am I doing wrong?