[otrs] TRying to search from BaseDN="" for top level search

I am tring to configure otrs to search from the top of my ldap tree so I have set $Self->{'Customer::AuthModule::LDAP::BaseDN'} = ""; But in the web interface I get <snip> Software error: Need CustomerUser->Params->BaseDN in Kernel/Config.pm at /var/www/otrs//Kernel/System/CustomerUser/LDAP.pm line 40. For help, please send mail to the webmaster (root@localhost), giving this error message and the time and date of the error. </snip> If I edit the line /var/www/otrs//Kernel/System/CustomerUser/LDAP.pm and explicitly specify the basedn as blank then everything works. What is the best way to specify a top level search for LDAP ?