ctools_object_cache data not returned from ctools_object_cache_get

I'm using the ctools object cache in an ajax callback to store data so it is available later in a Solr hook. The data is stored fine in the table when I call ctools_object_cache_set(), but when I retrieve the data with ctools_object_cache_get(), I just get an empty object. In stepping through the ctools_object_cache_set() code, it chokes when it tries to unserialize the data. Here is the code I'm using to serialize it:

  foreach ($form_state['values'] as $id => $tids) {
    if (substr($id, 0, 6) == 'vocab_') {
      $vid = substr($id, 6);
      $filters = new stdClass();
      foreach ($tids as $tid) {
        if ($tid) {
          $filters->{$tid} = $tid;
        }
      }
      // Cache the object in ctools cache so it's available later.
      ctools_include('object-cache');
      ctools_object_cache_set('solr_filters', $id, $filters);
    }
  }

Is there something I'm doing in creating my object that's causing the problem? This is my first time using ctools object caching, so there's always that possibility.

Thanks.