ice:panelCollapsible: images not shown when header facet's content is not surrounded by div

the images PnlHdr_collapsed.gif and PnlHdr_down.gif are not shown in the panelCollapsible's header when the header facet of this component doesn't enclose its content within a div like:

<ice:panelCollapsible style="width:500px;"
    expanded="true">
     <f:facet name="header">
         <div>
<ice:outputText value="Collapsible Panel" />
</div>
     </f:facet>
     <ice:panelGroup style="height:350px;">
<ice:outputText value="Some content..." />
     </ice:panelGroup>
</ice:panelCollapsible>

Removing the div tags causes the images to disappear.