Lastpost link not working correctly.

<phrase 1="member.php?find=lastposter&amp;t=$thread[threadid]" 2="$thread[lastposter]">$vbphrase[by_x]</phrase>  <a href="showthread.php?$session[sessionurl]p=$thread[lastpostid]$thread[highlight]#post$thread[lastpostid]"><img class="inlineimg" src="$stylevar[imgdir_button]/lastpost.gif" alt="$vbphrase[go_to_last_post]" border="0" /></a>

That's the snippet in question from my threadbit. I'd really rather just fix the issue than blanket-revert my templates, as suggested on vb.com (to which I was subsequently ignored).

It works if I hit it from viewing a particular forum, such as:

http://www.sevenstring.org/forum/forumdisplay.php?f=7

However if I'm just on the forum index, it does not:

http://www.sevenstring.org/forum/index.php

Problem is that it's not actually taking anyone to the last post, it's just going to the first post in the thread. Any insight would be appreciated. Thanks!