Works except search pages

This Plugin works perfect except for search resulting pages. Initial situation: Example Menu course\searchpage (course is a custom-post-type with separate permalink). So searchpage was a submenu of course. When I click to searchpage, the submenu (with searchpage item) was shown. Now I hit a search, so the URL changes from http://www.example.com/searchpage/ to http://www.examle.com/searchpage/?s=searchterm
You see it’s not a subsite of searchpage, just a parameter.
But now the submenu was empty.

Is there a way to fix this?