Internal error

From cnl wiki

Detected bug in an extension! Hook efCategoryTreeParserOutput failed to return a value; should return true to continue hook processing or false to abort.

Backtrace:

#0 /ext/opt/tw/htdocs/html/tw.rpi.edu/proj/cnl.wiki/includes/OutputPage.php(445): wfRunHooks('OutputPageParse...', Array)
#1 /ext/opt/tw/htdocs/html/tw.rpi.edu/proj/cnl.wiki/includes/OutputPage.php(453): OutputPage->addParserOutputNoText(Object(ParserOutput))
#2 /ext/opt/tw/htdocs/html/tw.rpi.edu/proj/cnl.wiki/includes/OutputPage.php(409): OutputPage->addParserOutput(Object(ParserOutput))
#3 /ext/opt/tw/htdocs/html/tw.rpi.edu/proj/cnl.wiki/includes/OutputPage.php(383): OutputPage->addWikiTextTitle('This is a list ...', Object(Title), true)
#4 /ext/opt/tw/htdocs/html/tw.rpi.edu/proj/cnl.wiki/includes/specials/SpecialAllmessages.php(58): OutputPage->addWikiText('This is a list ...')
#5 [internal function]: wfSpecialAllmessages(NULL, Object(SpecialPage))
#6 /ext/opt/tw/htdocs/html/tw.rpi.edu/proj/cnl.wiki/includes/SpecialPage.php(748): call_user_func('wfSpecialAllmes...', NULL, Object(SpecialPage))
#7 /ext/opt/tw/htdocs/html/tw.rpi.edu/proj/cnl.wiki/includes/SpecialPage.php(534): SpecialPage->execute(NULL)
#8 /ext/opt/tw/htdocs/html/tw.rpi.edu/proj/cnl.wiki/includes/Wiki.php(224): SpecialPage::executePath(Object(Title))
#9 /ext/opt/tw/htdocs/html/tw.rpi.edu/proj/cnl.wiki/includes/Wiki.php(55): MediaWiki->initializeSpecialCases(Object(Title), Object(OutputPage), Object(WebRequest))
#10 /ext/opt/tw/htdocs/html/tw.rpi.edu/proj/cnl.wiki/index.php(93): MediaWiki->initialize(Object(Title), NULL, Object(OutputPage), Object(User), Object(WebRequest))
#11 {main}