Script to set reference field to empty

I have a script that attempts to set link a child record to a parent table based on an expression.  If the child record doesn't meet the crieteria, it sets the link to "empty".  Similar to:

if childRecordName = "Test" then
    refParentRecord := 1
else
    refParentRecord := null
end

But the script gives an error stating, "Expression must return a number or record id: null at line ... etc."

How can I set a reference field to Null within a script's If / Else statement?