Simple Search - no results problems

Here is my simple search tag: {exp:search:simple_form channel="md|md17|md18" result_page="md18/search" results="10" search_in="everywhere" no_result_page="md18/search/noresults"}

It doesn’t matter what I enter for the no_result_page or if I omit it altogether. If no results are available, i always get the error page “The action you have requested is invalid.”

Example at http://www.4sonline.org/md.

Thanks.