The operand is not compatible with the type of the function.

Hi Friends;

Here below is the code for my Display method, it gives me an error message "The operand is not compatible with the type of the function".  Here my motive is show automatic invoice reference field in vendor invoice journal

Display Real Invoicereference()
{

purchTable purchTable;

PurchParmTable PurchParmTable;


while select purchTable
where purchTable.purchID == vendInvoiceJour.purchID
join PurchParmTable
where PurchParmTable.PurchId == purchTable.purchId &&
PurchParmTable.InvoiceAccount == vendInvoiceJour.InvoiceAccount 

return (PurchParmTable.AutomaticInvoiceReference) ;
}

Can some one tells me exactly what did I do wrong? Thanks for your Time.