Author of the post in hidden field

Hi,
i’using this code in a hidden field to take the email of the post author in which my form is compiled. It seems that miss a little thing but i don’t understand what Is anyone can help me?
Thanks a lot

function my_filter_function( $data, $field_id ){
  // $data will contain all of the field settings that have been saved for this field.
  // Let's change the default value of the field if it has an ID of 3
  if( $field_id == 15 ){
    global $post;
    $data['default_value'] = the_author_meta('user_email');
  }
  return $data;
}
add_filter( 'ninja_forms_field', 'my_filter_function', 10, 2 );