post_type restriction on wp_query doesn't work?

I’m flummoxed. Although I thought I implemented a post type restriction correctly in WP_Query, I’m still getting returned posts that are NOT in my post_type.

Here’s the sitch:

I have a custom post type (“providers”) created through Custom Post Type UI version 0.8.2 .

I want to display *only* providers (not posts) that match a search term stored in $providername. Here’s my code. Note that I have Relevanssi enabled, which is why there’s some conditional code, but whether it’s activated or not, I’m still getting a search result that includes posts as well as providers. I want providers only, and not other post types, in this case the standard WP “post”. What am I missing?

if (!empty($providername)) {
          $args = array(
            'post_type' => 'providers',
            's' => $providername,
            'posts_per_page' => -1
          );
          if (function_exists(relevanssi_do_query)) {
            $blog_query = new WP_Query($args);
            relevanssi_do_query($blog_query);
          }
          else {
            $blog_query = new WP_Query($args);
        }
    }