ldap_add: Invalid syntax (21)

[Graeme Gemmill <graeme@gemmill.name>]

Sorry, but after a couple of hours head-scratching, I need some help 
from the experts.
Environment: Mandriva Linux 2011_x64, OpenLDAP openldap-2.4.33 (also 
jxplorer).
I'm trying to process a large LDIF file produced by Thunderbird. My 
slapd.conf file contains
include        /usr/local/etc/openldap/schema/core.schema
include        /usr/local/etc/openldap/schema/cosine.schema
include        /usr/local/etc/openldap/schema/inetorgperson.schema
include        /usr/local/etc/openldap/schema/openldap.schema
include        /usr/local/etc/openldap/schema/mozilla.schema
include        /usr/local/etc/openldap/schema/misc.schema

I've progresses to the point where two organizational units have been 
created, Personal and NCI.
I get the error when processing the first person in the LDIF file:

dn: cn=Chris 
Smith,mail=chris.smith@ccb-insight.emv1.net,ou=Personal,dc=gemmills,dc=name
objectclass: top
objectclass: person
objectclass: organizationalPerson
objectclass: inetOrgPerson
objectclass: mozillaAbPersonAlpha
cn: Chris Smith
mail: chris.smith@ccb-insight.emv1.net
#modifytimestamp: 1314088910

The full error message is ldap_add: Invalid syntax (21)
        additional info: objectclass: value #4 invalid per syntax,
which points to a problem with the mozillaABPersonAlpha object.

here is the log output:
add objectclass:
    top
    person
    organizationalPerson
    inetOrgPerson
    mozillaAbPersonAlpha
add cn:
    Chris Smith
add mail:
    chris.smith@ccb-insight.emv1.net
adding new entry "cn=Chris 
Smith,mail=chris.smith@ccb-insight.emv1.net,ou=Personal,dc=gemmills,dc=name"

I can't see why either of these attributes should give a problem.
As a side issue: is it significant that KWrite doesn't colour the 
"objectclass: mozillaAbPersonAlpha" line in the LDIF file the same as 
the other objectclass lines?
Your comments/help will be gratefully received.
Graeme