[sql2005] update using case statement

[jlbantang]

Hi there, need extra head once more.

Im updating the dlvrystatus of purchase item against certain condition. The value will be determine through CASE statement. Note that the 2nd and 3rd condition work.

In the code I want to return PENDING for purchase item without deliveries(tblmaterialreceiptdetails).

update tbllpodetails  set dlvrystatus=(
	case	
		--assign Pending for po without deliveries
		when  is null(select sum(b.qty)
			from	tblmaterialreceiptdetails b
			where b.fk_lpodetailid=tbllpodetails.pk_lpodetailid)
			then 'Pending'

	 --assign Received for lpo without deliveries
		when tbllpodetails.qty=(select sum(b.qty)
			from	tblmaterialreceiptdetails b
			where b.fk_lpodetailid=tbllpodetails.pk_lpodetailid) 
			then 'Received'
	else 
		'Partial'
	end
)

Error:
Msg 156, Level 15, State 1, Line 11
Incorrect syntax near the keyword 'is'.
Msg 156, Level 15, State 1, Line 14
Incorrect syntax near the keyword 'then'.
Msg 156, Level 15, State 1, Line 20
Incorrect syntax near the keyword 'then'.

Thanks.