Fax can't have equality index?

[Min <pingminadmin@gmail.com>]

Hello,

I have the fax in my sldap.conf as the following:

index facsimileTelephoneNumber          eq,pres,sub
index telephoneNumber                   eq,pres,sub

But slaptest will report error as:

equality index of attribute "facsimileTelephoneNumber" disallowed
slaptest: bad configuration file!

Any help will be appreciated.

Thanks,