add function by VBA

Hi
I tried to implement the tip from http://www.exceltip.com/show_tip/Cus...ction/147.html
And got syntax error .
What might be the problem?
I did the entire step by steps:
Build a module:

Function SalesTax(Total_Invoice, Tax_percentage) As Single
SalesTax = Total_Invoice – Total_invoice / (1+Tax_percentage)
End Function

Saved it as xlsm
The function was build and can be operate in the excel sheet.
( And give the error )