User Defined Functions Error ....

Hi Friends,
I am working on Project Using VB 6.0 & MS Access 2000..
I am using ADO for Database connection...

Problem is while Executing the below qry, I am getting error...

Qury Name: qry_GenerateInvoice

In Access:-
"SELECT IIf(IsNull(Max([Sales]![InvoiceNo])),fnFormatDate(Date()),Max([Sales]![InvoiceNo]))+1 AS Expr1 FROM Sales WHERE (((Sales.InvoiceNo)>fnFormatDate(Date())));"


In query, fnFormatDate is Function written in Access Module...


While executing the query from VB, I am getting error Invalid SQL Statement expecting INSERT, UPDDATE OR SELECT STATEMENT.... While I further working on this, I am getting error - UNDEFINED FUNCTION 'fnFormatDate'EXPRESSION

How to solve this problem?
Mainly I need solution that whether I call functions of Access Module using Select Statement from VB 6.0

FYI: This Query work fine in Access
Thanxs in advance..