On demo.piwik.org's API page, and selecting RSS or XML output:
Notice: Object of class Piwik_DataTable_Array could not be converted to int in /path/to/piwik/core/Piwik.php on line 1023
the problem occurs if more than one value is requested.
Piwik::getPrettyTimeFromSeconds should be applied using a filter instead of a direct use.
(In ) fixes #1576 use filter if more than one value is requested