Orbeon: set initial value for fields if a check box is checked

I have multiple address fields: Street_number, Street_name, City, Country, State, Postal_code. Suppose i filled those in the form previously

Suppose that later on in the form if I check a box, some of those previously filled fields concatenate into one address field

i am setting the "initial value" of this address field to the code below:

if($concat_checkbox=true())
then concat($Street_num, ' ', $Street_name, , ' ',$City,' ',$State,' ',  $Country,' ',$Postal_code)

Orbeon is complaining: "Incorrect XPath expression", what is the correct way to use conditions in this case?