I invoiced a taxable customer last month but sales tax was not added to the invoice. Now I need to send the customer an additional invoice for sales tax only. 

I cannot use the tax as an item because it gives me an error that says "This sales tax item has been specified in the Tax field or is a member of the sales tax group being used in the tax field"

How do I create an invoice that is only for sales tax only or is there a better way to do this?