Why do I get an error during tracker incremental load job?

An error (as illustrated below) shows up during the tracker incremental load job in case there is no data to harvest for certain fields.

You can ignore such errors that have no impact on the incremental load job's success or failure.

2015-11-02 02:30:05,547 ERROR [TrackerIncrementalJob] SetVar - We can not field
 [date_to_harvest_teams] in the previous job entry result!
2015-11-02 02:30:05,940 ERROR [TrackerIncrementalJob] SetVar - We can not field
 [date_to_harvest_dependency] in the previous job entry result!
2015-11-02 02:30:06,399 ERROR [TrackerIncrementalJob] SetVar - We can not field
 [date_to_harvest] in the previous job entry result!
2015-11-02 02:30:11,278 ERROR [TrackerIncrementalJob] SetVar - We can not field
 [date_created] in the previous job entry result!
2015-11-02 02:30:11,493 ERROR [TrackerIncrementalJob] SetVar - We can not field
 [date_membership_removed] in the previous job entry result!