warning when I use formatter in a field and try to save formatter again

Warning: implode(): Invalid arguments passed in custom_formatters_engine_php_settings_form_submit() (line 66 of /var/www/html/sites/all/modules/custom_formatters/engines/php.inc).

error inside function below : when I use formatter function( form_state['values']['field_types'] ) no contain the array(only one text value)

function custom_formatters_engine_php_settings_form_submit($form, &$form_state) {

$form_state['values']['field_types'] = implode(', ', $form_state['values']['field_types']);

}

To reproduse issue I use text field