First argument should be an array, other array errors: lines 402,403 of xmlsitemap.admin.inc

I'm on a vanilla d6 install with XML Sitemap (and all included modules) turned on. When I go to node/add/page, I get:

* warning: in_array() [function.in-array]: Wrong datatype for second argument in {path to modules dir}/xmlsitemap/xmlsitemap.admin.inc on line 402.
* warning: array_unshift() [function.array-unshift]: The first argument should be an array in {path to modules dir}/xmlsitemap/xmlsitemap.admin.inc on line 403.

Taking a look at those lines, I see:

401   // Add the submit handler to adjust the default values if selected.
402   if (!in_array('xmlsitemap_process_form_link_options', $form['#submit'])) {
403     array_unshift($form['#submit'], 'xmlsitemap_process_form_link_options');
404   }

If I change it to the code below, all is fine. Not sure how this would affect other functionality, but it seems to work.

401   // Add the submit handler to adjust the default values if selected.
402   if (is_array($form['#submit'])) {
403     if (!in_array('xmlsitemap_process_form_link_options', $form['#submit'])) {
404       array_unshift($form['#submit'], 'xmlsitemap_process_form_link_options');
405     }
406   }

See patch, attached.