Call to undefined function apachesolr_search_noresults()

After upgrading to 6.x.1.11, the search page would no longer display whenever no results were found. This was caused by:

Call to undefined function apachesolr_search_noresults()

Which is called on line 197 of the module:

 // If we are searching with apache solr, use the no results text
            // from the apachesolr_search module (#1127412).
            if (module_exists('apachesolr_search')) {
              $results = apachesolr_search_noresults();

I corrected this by changing apachesolr_search_noresults() to theme_apachesolr_search_noresults().
I'm running 6.x-2.0-beta5 of the apache solr integration module. I didn't notice the error until upgrading search404 but it certainly could have been present.