How to fix Scaffolding evaluate data expression problem of /zero

[Herny C]

Hi, Guys. 

I use an expression in Evaluate data macro(Scaffolding Forms & Templates).

( 1 - (${pdb_target_cost} / ${pdb_target_price})) * 100

 when the ${pdb_target_price} is null, I will get the err in below.

How can I do to fix it?  Thanks.