extending posixAccount...?

["paul.reilly@tcd.ie" <pareilly@tcd.ie>]

I've registered for an OID, and I'm trying to extend the nis.schema
posixAccount class, but keep getting this error on starting OpenLDAP...

line 194: user-defined ObjectClass includes operational attributes: "posixAccount"


objectclass ( 1.3.6.1.4.1.13800.2.2.100 NAME 'tcdAcount'
        SUP posixAccount
        DESC 'TCD Person'
        MUST (  tcdFirstName $ tcdSurname $ tcdEmail $
                macosxUnixShell $ macosxHomeDir $ macosxNetHome         )
        MAY  (  tcdMiddleName $ tcdTitle $ tcdDepartmentName $		))


Yet, if I make my class a subclass of inetOrgPerson or top etc it works.
Whats special about posixAccount that I can't subclass it?!

Paul