Calculated column add values from mutiple columns with IF statements

I tried to added multiple invoice amounts (if they are not paid) to the total amount due (calculated) column with the following formula:  

=IF([InvoicePaid]=FALSE,[InvoiceAmount],0)+
IF([InvoicePaid2]=FALSE,[InvoiceAmount2],0)

I got an error when I save the formula. If I change the second IF statement to a number such as 5, then the formula works fine. I hope someone can tell me what is the correct way to add multiple IF statement together.

Thanks a bunch for your advise.