NoMethodError：未定义的方法`in_time_zone＆＃39;为零：NilClass

Question

我正在尝试将数据插入到我的TimeSlot表中。加班我运行代码，我得到这个错误＆＃34; NoMethodError：未定义的方法`in_time_zone＆＃39;为零：NilClass＆＃34;

以下是插入数据的代码：

 starts_at= begin_date
 time_slot=TimeSlot.create( starts_at: '',
                                  length: 15,
                                  provider_id: '',
                                  practice_location_id: '',
                                  practice_id: '',
                                  day_of_the_week: ' ',
                                  timezone: "Central Time (US & Canada)",
                                  hour_of_the_day: 1,
                                  health_post_id: time_slot_start_health_post_id,
                                  deleted_at: 'NULL',
                                   health_post_url: url,
                                  cached_reason_for_visits_list: "8~Allergic Asthma|9~Allergic Cough|10~Allergic Eye Problems|7~Allergy Consultation|5~Discharge (ER) Follow Up Visit|6~Discharge (Hospital) Follow Up Visit|11~Drug Allergy|12~Eczema|3~Follow Up – Established Patient|13~Food Intolerance|2~General Consultation|14~Hay Fever|15~Hives|1~New Patient|16~Poison Ivy|4~Sick Visit – Established Patient|17~Skin Rashes",
                                  type_of_patient: "any_test")

以下是模型

中time_zone的功能

   def local_starts_at
     set_timezone unless self.timezone

     @local_starts_at ||= self.starts_at.in_time_zone(self.timezone)
   end

请帮忙。