Dropdown help 4.7

I'm getting issues with generating a taxonomy dropdown in 4.7
- I have two issues

One is that the code outputs the correct dropdown, but generates this in the logs.

implode(): Bad arguments. in /home/drupal/public_html/includes/form.inc on line 291.

The second is that the #default_value isn't getting selected.

(They could be related).

Anyone care to look, and see what I'm doing wrong?

function countrydata_dropdown ($current_country) {
	//Show country as select dropdown
	$countries = taxonomy_get_children(0,variable_get('country_vocabulary', false));
    $countries_array = array();
    $countries_array[0] = t('Country');
    foreach ($countries as $country) {
      $countries_array[$country->tid] = $country->name;
    }

   $form['country'] = array(
  '#type' => 'select',
  '#title' => '',
  '#name' => 'countryselect',
  '#id' => 'countryselect',
  '#default_value' => $current_country,
  '#options' => $countries_array,
  '#description' => '',
  '#attributes' => array('onChange' => 'top.location.href=document.getElementById("countryselect").options[document.getElementById("countryselect").selectedIndex].value')
);
    //Output the form
	$output= form_render($form[country]);
	return ($output);
}