Specifying sort order breaks search results on group page

[seanh]

Create a group, create some datasets and add them to the group, then view the group's page with a sort-order specified in the URL, e.g. /group/foo?q=&sort=title_string+asc, no datasets are displayed even though there are datasets in the group that match the search query. Remove the sort order from the URL and the datasets will appear.