XHTML validation error caused by form action tag empty

When validating the viewtopic.php page with the W3C Validator I get an error regarding the form action tag of the jumpbox (Messages order). The error is caused by the fact that the variable S_JUMPBOX_ACTION is empty in that page and this behaviour is not allowed by xhtml.

File: jumpbox.html

<form method="post" id="jumpbox" action="{S_JUMPBOX_ACTION}" onsubmit="if(document.jumpbox.f.value == -1){return false;}">