Show only “Pages” – not posts – on search page

When I have this URL:

http://localhost/website/?post_type=post&s=lorem

It shows me only the posts which have the word “lorem”. Now I want to this with pages:

http://localhost/website/?post_type=page&s=lorem

I still get the posts. Any Idea? (With other custom post types like “portfolio” it works, too).