Internal error in ApiFormatFeedWrapper::execute: Invalid feed class/item

Backtrace:

#0 /web/htdocs/www.enciclopediadeldoppiaggio.it/home/includes/api/ApiBase.php(1241): wfDebugDieBacktrace('Internal error ...')
#1 /web/htdocs/www.enciclopediadeldoppiaggio.it/home/includes/api/ApiFormatBase.php(368): ApiBase::dieDebug('ApiFormatFeedWr...', 'Invalid feed cl...')
#2 /web/htdocs/www.enciclopediadeldoppiaggio.it/home/includes/api/ApiMain.php(728): ApiFormatFeedWrapper->execute()
#3 /web/htdocs/www.enciclopediadeldoppiaggio.it/home/includes/api/ApiMain.php(387): ApiMain->printResult(true)
#4 /web/htdocs/www.enciclopediadeldoppiaggio.it/home/includes/api/ApiMain.php(342): ApiMain->executeActionWithErrorHandling()
#5 /web/htdocs/www.enciclopediadeldoppiaggio.it/home/api.php(115): ApiMain->execute()
#6 {main}