Samba4 primaryGroupID problem

[steve]

Hi
user steve2
memberOf: cn=laser,cn=Users,dc=foo,dc=bar
primaryGroupID: 513


After setting primaryGroupID for steve2 to 'laser' by replacing the 
primaryGroupID 513 with that of 'laser', 1108 in this case, the memberOf 
attribute remains.

Reverting steve2 to primaryGroupID 513 and then attempting to remove the 
group membership:

samba-tool group removemembers laser steve2
completes but the attribute remains.

using ldbedit in an attempt to remove it gives:
failed to modify CN=steve2,CN=Users,DC=polop,DC=site - LDAP error 53 
LDAP_UNWILLING_TO_PERFORM - <00002035: objectclass_attrs: attribute 
'memberOf' on entry 'CN=steve2,CN=Users,DC=polop,DC=site' must not be 
modified directly, it is a linked attribute> <>

Any ideas?
Cheers,
Steve