ROR + Error For nil:NilClass In Rake Task

Question

In my Rake Command I am calling XML to retrieve data. If person_id is valid, then it saved. But I am getting error.

if !@case.person_id.nil?
    @project.team_members << @person
 end

Error :: rake aborted! undefined method `team_members' for nil:NilClass

Please Suggest Some Thing !!!