Custom Formula Field in CRM Invoice Module (Syntax error)

Custom Formula Field in CRM Invoice Module (Syntax error)

Hello!

I am trying to make a simple custom formula field in the invoice module of Zoho CRM. I'm trying to get the result of multiplying two custom number fields. Here is the senario:

Custom field one: Pick List Called "Cost1"
Custom field two: Pick List Called "Qty1"

Custom formula: ${Invoices.Cost1}*${Invoices.Qty1}

It comes up with a Syntax error. What am I doing wrong?! Is this possible to do?

Thank you for your help!