How do you load a select field with pre-selected items when the Chosen module is installed/enabled?
Your select field is currently only accepting one selected option. This is why it won't allow an array as a default value. (It expects a string or an integer).
The following select field allows more than a selected option.
$form['list_info']['my_list'] = array(
  '#title' =&gt; t('Select a list to manage:'),
  '#type' =&gt; 'select',
  '#options' =&gt; $my_list,
  '#default_value' =&gt; $my_list,
  '#multiple' =&gt; TRUE,
);