PM preview background colour not being set correctly

In posting_preview.html the PM background colour "IF" statement doesn't work.

This is the code:

<div class="post <!-- IF S_PRIVMSGS --> pm<!-- ELSE --> bg2" id="preview<!-- ENDIF -->">  

Yet when in PM preview mode the class being applied is bg2 rather than pm.

<div id="preview" class="post bg2">  

Manually changing the class to "post pm" in Firebug results in the correct white background so the css class is present and correct.