When trying to insert the following text into a page:
{{ShoutWiki Calendar:|position=Right|start=1|lang=user|highlightedDays=1 9 17}}

And previewing, I get this error:

Non-string key given
Backtrace:
#0 /var/www/MediaWiki/Git/core/includes/Message.php(666): MessageCache->get(NULL, true, Object(Language))
#1 /var/www/MediaWiki/Git/core/includes/Message.php(431): Message->fetchMessage()
#2 /var/www/MediaWiki/Git/core/includes/Message.php(503): Message->toString()
#3 /var/www/MediaWiki/Git/core/languages/Language.php(814): Message->text()
#4 /var/www/MediaWiki/Git/core/languages/Language.php(887): Language->getMessageFromDB(NULL)
#5 /var/www/MediaWiki/Git/extensions/Calendar/Calendar.php(133): Language->getWeekdayAbbreviation(0)
#6 /var/www/MediaWiki/Git/extensions/Calendar/Calendar.php(234): CalendarTable->getWeekdayAbbrev(0, 0)
#7 /var/www/MediaWiki/Git/extensions/Calendar/Calendar.php(475): CalendarTable->buildTable()
#8 [internal function]: wfCalendar(Object(Parser), '', 'position=Right', 'start=1', 'lang=user', 'highlightedDays...')
#9 /var/www/MediaWiki/Git/core/includes/parser/Parser.php(3278): call_user_func_array('wfCalendar', Array)
#10 /var/www/MediaWiki/Git/core/includes/parser/Preprocessor_DOM.php(1084): Parser->braceSubstitution(Array, Object(PPFrame_DOM))
#11 /var/www/MediaWiki/Git/core/includes/parser/Parser.php(3065): PPFrame_DOM->expand(Object(PPNode_DOM), 0)
#12 /var/www/MediaWiki/Git/core/includes/parser/Parser.php(1150): Parser->replaceVariables(''''MediaWiki ha...')
#13 /var/www/MediaWiki/Git/core/includes/parser/Parser.php(378): Parser->internalParse(''''MediaWiki ha...')
#14 /var/www/MediaWiki/Git/core/includes/content/WikitextContent.php(290): Parser->parse(''''MediaWiki ha...', Object(Title), Object(ParserOptions), true, true, NULL)
#15 /var/www/MediaWiki/Git/core/includes/EditPage.php(2962): WikitextContent->getParserOutput(Object(Title), NULL, Object(ParserOptions))
#16 /var/www/MediaWiki/Git/core/includes/EditPage.php(1980): EditPage->getPreviewText()
#17 /var/www/MediaWiki/Git/core/includes/EditPage.php(418): EditPage->showEditForm()
#18 /var/www/MediaWiki/Git/core/includes/actions/EditAction.php(51): EditPage->edit()
#19 /var/www/MediaWiki/Git/core/includes/actions/EditAction.php(71): EditAction->show()
#20 /var/www/MediaWiki/Git/core/includes/Wiki.php(429): SubmitAction->show()
#21 /var/www/MediaWiki/Git/core/includes/Wiki.php(305): MediaWiki->performAction(Object(Article), Object(Title))
#22 /var/www/MediaWiki/Git/core/includes/Wiki.php(555): MediaWiki->performRequest()
#23 /var/www/MediaWiki/Git/core/includes/Wiki.php(448): MediaWiki->main()
#24 /var/www/MediaWiki/Git/core/index.php(59): MediaWiki->run()
#25 {main}

Version: unspecified
Severity: major