IntegrityError when updating user

Hello! I want to integrate askbot to my django project. When I'm trying to update user information using admin interface, i'm getting this exception:

update or delete on table "auth_user_groups"
violates foreign key constraint "authusergroups_ptr_id_refs_id_7859c44e"
on table "askbot_groupmembership"

What are the reasons this can happen?