refint overlay not working (revisited)

Last time I have asked for any suggestion why refint is not working 
(http://www.openldap.org/lists/openldap-software/200910/msg00112.html). Today I have revisited this 
issue, turned on logging, and found some strange error logs:

Nov  5 12:03:44 localhost slapd[26252]: refint_search_cb <NOTHING>
Nov  5 12:03:44 localhost slapd[26252]: bdb_dn2entry("cn=bbb,ou=emailaliases,dc=iwt,dc=local")
Nov  5 12:03:44 localhost slapd[26252]: bdb_modify_internal: 0x0000006c: 
cn=bbb,ou=emailAliases,dc=example,dc=com
Nov  5 12:03:44 localhost slapd[26252]: hdb_modify: modify failed (50)
Nov  5 12:03:44 localhost slapd[26252]: send_ldap_result: conn=-1 op=0 p=0
Nov  5 12:03:44 localhost slapd[26252]: refint_repair: dependent modify failed: 50

After some research, I found a hint from 
http://www.openldap.org/lists/openldap-technical/200909/msg00022.html (although seems unrelated to 
refint overlay). Refint overlay works immediately after rootdn is configured.

I have read the documentation afterwards, and seems that the documentation doesn't mention refint 
overlay works only if rootdn is configured. I would like to ask if it is (undocumented) requirement
or a software bug?