Magic word 'mycustomvar1' not found

Backtrace:

#0 /usr/share/mediawiki/includes/MagicWord.php(244): Language->getMagic(Object(MagicWord))
#1 /usr/share/mediawiki/includes/MagicWord.php(197): MagicWord->load('mycustomvar1')
#2 /usr/share/mediawiki/includes/MagicWord.php(522): MagicWord::get('mycustomvar1')
#3 /usr/share/mediawiki/includes/MagicWord.php(647): MagicWordArray->getHash()
#4 /usr/share/mediawiki/includes/parser/Parser.php(2741): MagicWordArray->matchStartToEnd('imageserver')
#5 /usr/share/mediawiki/includes/parser/Preprocessor_DOM.php(959): Parser->braceSubstitution(Array, Object(PPFrame_DOM))
#6 /usr/share/mediawiki/includes/parser/Parser.php(2632): PPFrame_DOM->expand(Object(PPNode_DOM), 0)
#7 /usr/share/mediawiki/includes/parser/Parser.php(875): Parser->replaceVariables(''''Images'''??F...')
#8 /usr/share/mediawiki/includes/parser/Parser.php(327): Parser->internalParse(''''Images'''??F...')
#9 [internal function]: Parser->parse(''''Images'''??F...', Object(Title), Object(ParserOptions), true, true, 9)
#10 /usr/share/mediawiki/includes/StubObject.php(58): call_user_func_array(Array, Array)
#11 /usr/share/mediawiki/includes/StubObject.php(76): StubObject->_call('parse', Array)
#12 /usr/share/mediawiki/includes/Article.php(3557): StubObject->__call('parse', Array)
#13 /usr/share/mediawiki/includes/Article.php(3557): StubObject->parse(''''Images'''??F...', Object(Title), Object(ParserOptions), true, true, 9)
#14 /usr/share/mediawiki/includes/Article.php(979): Article->outputWikiText(''''Images'''??F...')
#15 /usr/share/mediawiki/includes/Wiki.php(450): Article->view()
#16 /usr/share/mediawiki/includes/Wiki.php(63): MediaWiki->performAction(Object(OutputPage), Object(Article), Object(Title), Object(User), Object(WebRequest))
#17 /usr/share/mediawiki/index.php(116): MediaWiki->initialize(Object(Title), Object(Article), Object(OutputPage), Object(User), Object(WebRequest))
#18 {main}