I am getting a similar error in creating an invoice.
I have two tables, one for descriptions, qty, price, and subtotal and one for Grand Subtotals, Tax, and Total.
Calculations are fine when multiplying quantity by price for subtotal. But when I try and retrieve a Sum(Subtotal1,Subtotal2,etc..) I get the same error. If I delete the calculations in the second table no more error.
Any ideas. Does the fact of two separate tables cause this?