Sync fails for FIM service with "Attribute "uid" is not present"

I have an MvExtension DLL that provisions new AD accounts. When I do a full sync from the FIM Service MA it fails with the error "Attribute "uid" is not present". Attribute uid is present in the MA Person attribute flows. A full sync from the SQL MA works. Is there something special about the FIM Service (portal) MA?

thanks,

Alistair