XF 2.2 Show HTML widget only to logged out visitors

We are trying to create a 'sign up' button on the sidebar for users who are not logged in and wish to hide it for anyone who is logged in. We tried to use this statement in the display condition box of the HTML widget:

<xf:if is="$xf.visitor.isMemberOf([1])"></xf:if>

But it isn't working. Any ideas? User group 1 is unregistered/unconfirmed guests.