Invalid version for use with report XML

[Roy Yu]

I'm new to AdWords and I'm using AdWords API  v201502, I'm trying to download the report, but I got a 400 back from the curl call and the response is "Invalid version for use with report XML".  I'm kind of lost currently, any help will be appreciated. 

string(0) ""

array(3) {

  [0] =>

  string(54) "!!!2|||-1|||Invalid version for use with report XML???"

  [1] =>

  string(2) "-1"

  [2] =>

  string(39) "Invalid version for use with report XML"

}

int(400)

PHP Fatal error:  Uncaught exception 'ReportDownloadException' with message 'Invalid version for use with report XML' in /www/roy-default/vendor/chegg/core/Chegg.Thirdparty/googleads-php-lib-master/src/Google/Api/Ads/AdWords/Util/ReportUtils.php