error with Invalid argument supplied for foreach()

warning: Invalid argument supplied for foreach() in /var/www/drupal/sites/all/modules/gmaps/modules/gmaps_user/gmaps_user.module on line 352.

lines consists of

if (is_null($serialize)) {
$serialize = array();
$schema = drupal_get_schema('gmaps_user_marker_option');
foreach ($schema['fields'] as $column => $info) { //line#352 is here
if ($info['serialize']) {
$serialize[] = $column;
}
}