ldap_add: Object class violation

["Missing Wang" <wang@lib.uibe.edu.cn>]

Hi,
I am newer of Openldap.
I add a initial entry like that 

ldapadd -D "cn=root, dc=uibe, dc=com" -W </usr/local/etc/openldap/myldif
the myldif like this

dn: dc=uibe, dc=com
dc: uibe
o: mycompany
objectclass: organization
objectclass: dcObject
dn: cn=root, dc=uibe, dc=com
cn: root
sn: root
objectclass: person 

then it succeed.
then I want to add the second entry 

ldapadd -f second.dif

the screen output like that:
ldap_add: Object class violation

second.dif:
dn: cn=john, dc=uibe, dc=com
objectClass: person
cn: john wang
mail: john@uibe.edu.cn

Please help me.