Search form generating W3C error

Hi,
Events Manager is generating a W3C error we need to resolve:

Error: Bad value .em-search-advanced:.em-search-form for attribute rel on element a: The string .em-search-advanced:.em-search-form is not a registered keyword.

<a href="#" class="em-toggle" rel=".em-search-advanced:.em-search-form">

Could you advise? Thanks in advance