[Samba] Unable to modify SID for "administrator"

[Steve A]

Whenever I try to change the samba password for administrator, I get the 
following error:

---
Unable to modify TDB passwd ! Error: Record does not exist
 occured while storing the RID index (RID_000001f4)
Failed to modify entry for user administrator.
Failed to modify password entry for user administrator
---

I deleted the administrator account from the database (I'm using tdbsam), 
and despite seeing the message "Unable to delete user administrator", it had 
actually been deleted (I checked with pdbedit -L)

I then added the administrator account using 'smbpasswd -a administrator'

I then try to change the SID of this account to -500, using pdbedit, and I 
get a similar error to the above:

---
Unable to modify TDB passwd ! Error: Record does not exist
 occured while storing the RID index (RID_000001f4)
Unable to modify entry!
---

All other accounts are absolutely fine.  Is there something special about 
administrator?

Many thanks,
Steve :)