concatenated parameters in python shutdown script not fully resolving

I have a couple of published parameters param1, and param2 and a private param (param3) which is concatenated from the published parameter :  $(param1)-$(param2)

 

 

if param1 is A

 

and param2 is B

 

I'm expecting param3 to be A-B

 

 

instead, in the python shutdown script

 

fme.macroValues['param3'] returns A-$(param2)

 

 

any ideas on how to get it to fully resolved.