diff --git a/mongodb_session/mongodb_session.module b/mongodb_session/mongodb_session.module
index f05a9f7..dc5fcfc 100644
--- a/mongodb_session/mongodb_session.module
+++ b/mongodb_session/mongodb_session.module
@@ -29,9 +29,11 @@ function mongodb_session_user_login($edit, $account) {
 function _mongodb_session_get_roles($account) {
   $roles = array();
   $roles[DRUPAL_AUTHENTICATED_RID] = 'authenticated user';
-  $result = db_query("SELECT r.rid, r.name, ur.uid FROM {role} r INNER JOIN {users_roles} ur ON ur.rid = r.rid WHERE ur.uid IN (:uids)", array(':uids' => array($account->uid)));
-  foreach ($result as $role) {
-    $roles[(int) $role->rid] = $role->name;
+  if (!empty($account->uid)) {
+    $result = db_query("SELECT r.rid, r.name, ur.uid FROM {role} r INNER JOIN {users_roles} ur ON ur.rid = r.rid WHERE ur.uid IN (:uids)", array(':uids' => array($account->uid)));
+    foreach ($result as $role) {
+      $roles[(int) $role->rid] = $role->name;
+    }
   }
   return $roles;
 }