SOLR Stats on date field -

im trying stats on date field results failing when retrieve using pysolr -

the problem sum field. date - 28129-09-01t23:59:59.999z - not in correct format , while formatting date, errors.

<lst name="orderdate_tdt"> <date name="min">2013-08-16t00:00:00z</date> <date name="max">2014-08-21t00:00:00z</date> <long name="count">588</long> <long name="missing">0</long> <date name="sum">28129-09-01t23:59:59.999z</date> <date name="mean">2014-06-28t08:36:44.081z</date> <double name="sumofsquares">-2.0737443528198062e20</double> <double name="stddev">nan</double> <lst name="facets"/> </lst>