[Zope] base url is missing port number

I am using the tool WebStress (http://www.paessler.com/) to test our Zope server performance, but I have a problem with the base url: WebStress gets as base url: <base href="http://195.xxx.yyy.zzz/" /> But IE gets (the correct url): <base href="http://195.xxx.yyy.zzz:8181/" /> The WebStress url is missing the port 8181. Why is this so? Juergen