Adding users to Group

Hi Guys, I am trying to add members to a group: XYZ. We have configured LDAP in portal. For some of the users, First name is empty and whole name is displayed as Last Name. Due to this, whenever I am trying to search user (to be added in the group) such users could not be found.
Is this a bug or can we do some settings to enable the search of user with Last name also?