need some help with pl-pgsql

have a function written in pgsql it runs just fine, except its doing
something really stupid.

The function runs just fine till this select statement

Select (wo_wipvalue/wo_qtyord)
into _TotalCost
from wo
where wo_id = pWoid ;

This goes to the work order header table to gets the current value
divided by current qty thats been completed so far, then sticks the
value into _TotalCost variable based on the parameter passed into
variable pWoid.

problem is it returns NULL which is impossible as i can manually run
the select statement and get the $1.589445

I have no idea how to fix this problem going forward???

and the database is 8.3.5 running Ubuntu 8.0 lts