TOC generated from Robohelp 6 cannot be viewed in Internet Explorer 11

Hi,

I have created Table of Contents in RH 6 and generated the WebHelp. But when I open the help files via an application's help button in IE11, somehow the TOC is not displayed on the left pane. However, when I use Mozilla FF 32.0.3, the TOC is displayed without any problem. Can you advise how can I solve the issue in IE11?

Thanks.

WF