Logout System Portlet

I put the logout link in my shell (from Portlet/System). If I’m login in my portal using WM login page the logout link works perfectly. But if I’m login thru an http authentification the logout link display an error in my shell. Here the error.

com.webmethods.portal.system.impl.URI cannot be cast to com.webmethods.portal.service.meta2.thing.IThingID

any idea ?