Result returned by dtoc(invoice_dt)) can have different format and size depending on some SET settings. There's function specifically designed to be used for indexing - DTOS() or DTOC(,1).

upper(accountno+client+DTOS(invoice_dt))

>
>Seems to work ok until I run the following code
>
>
>Select Invoicestack
>Scan
>replace client with clients.client
>endscan
>
>I get the error 'Error building key for index'
>l:\global\account\invoicestack.cdx tag Invoiceget
>
>Invoiceget is indexed on upper(accountno+client+dtoc(invoice_dt))
>
>Do I need to rebuild all indexes in VFP9 before continuing?
>
>Colin