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

Backtrace:

#0 /var/www/testwiki/public/w/includes/api/ApiBase.php(1300): wfDebugDieBacktrace('Internal error ...')
#1 /var/www/testwiki/public/w/includes/api/ApiFormatBase.php(378): ApiBase::dieDebug('ApiFormatFeedWr...', 'Invalid feed cl...')
#2 /var/www/testwiki/public/w/includes/api/ApiMain.php(741): ApiFormatFeedWrapper->execute()
#3 /var/www/testwiki/public/w/includes/api/ApiMain.php(389): ApiMain->printResult(true)
#4 /var/www/testwiki/public/w/includes/api/ApiMain.php(344): ApiMain->executeActionWithErrorHandling()
#5 /var/www/testwiki/public/w/api.php(117): ApiMain->execute()
#6 {main}