BLS nullnumber assignment

Hi, I'm having difficulty assigning a nullnumber to a transaction's local property.

First I generate a local propery xml document. I then need to do a calculation with the content of this xml file. If one of the inputs to the calculation is a "" or NA, then I want the output to be a nullnumber ie: NA.

This logic is done using:

if(

Local.ReturnedDocXML{/Rowsets/Rowset/Row[./SubItem=="#Local.Rpt_ClosingStock_Measured#"]/Pt} !="NA"

and

Local.ReturnedDocXML{/Rowsets/Rowset/Row[./SubItem=="#Local.Rpt_ClosingStock_Measured#"]/Pt} !="",

Local.ReturnedDocXML{/Rowsets/Rowset/Row[./SubItem=="#Local.Rpt_ClosingStock_Measured#"]/Pt}

-

Local.ReturnedDocXML{/Rowsets/Rowset/Row[./SubItem=="#Local.Rpt_ClosingStock_System#"]/Pt} ,

nullnumber )

The output of this logic is assigned to a local property of type double. If the condition occurs where the value should be a nullnumber, the local property gets a value of 0. If I have a tracer action block with the same logic in the message link, the output is the NA as expected. However the local property's value is still 0.

Any ideas about this. I'm assuming that nullnumber is not shown as a 0. It's normally a "---" in the grid or an NA or "" in the xml output from what I can remember.