Error: Data structure returned is not convertible in the requested format: Only integer keys supported for array columns on base level. Unsupported string 'totals' found for row 'array (
  'nb_pageviews' => '4',
  'nb_visits' => '3',
  'nb_actions' => '4',
  'revenue' => '0',
  'nb_visits_lastdate' => 59.0,
)'. Try to call this method with the parameters '&format=original&serialize=1'; you will get the original php data structure serialized.