diff --git a/NetBiblio.php b/NetBiblio.php
index d2c7268..dbe6d50 100644
--- a/NetBiblio.php
+++ b/NetBiblio.php
@@ -214,6 +214,8 @@ class NetBiblio extends WebService
 
     public function FindBooks($codes)
     {
+        $this->CheckSession();
+
         $codes = json_decode($codes);
         $bs = new BookSearch();
         $bs->addQuery('code:('.implode(' OR ', $codes).')', null, false);
@@ -364,6 +366,8 @@ class NetBiblio extends WebService
 
     public function FindBook($code)
     {
+        $this->CheckSession();
+
         $bs = new BookSearch();
         $bs->addQuery($code, 'code');
         $results = $bs->getResults(0, 1);