Problem with case statement in sql procedure.

I have the following sql procedure with a case statement on the b.originalamt column.  Without the case the answer is null.  But with the statement it acts like it isn't null.  

select OccuSeqNo,RTRIM(a.propertyid)+'-'+RTRIM(a.BldgId)+'-'+RTRIM(a.unitid)+'-'+rtrim(a.resiid) as fullid, case b.OriginalAmt when null then 0 else b.OriginalAmt end as  originalamt,c.resistatus,
cast(c.LeaseBegDate as DATE) as leaseBegin,cast(c.LeaseEndDate as DATE) as leaseEnd
from TAG_eSite.dbo.occupantheader a
left  outer join CollectionSystem.dbo.Collection_Master as b on RTRIM(a.propertyid)+'-'+RTRIM(a.BldgId)+'-'+RTRIM(a.unitid)+'-'+rtrim(a.resiid) = b.fullid
right outer join TAG_esite.dbo.lease c on a.PropertyId=c.PropertyId and a.BldgId=c.BldgId and a.UnitId=c.UnitId and a.ResiId = c.ResiId

where OccuLastName = @LName and OccuFirstName = @FName and Birthdate=@Bdate and RIGHT(OccuSSN,4)=@SSNum and c.ResiStatus<>'X'
END

How can I fix this.

thank you