diff --git a/book.php b/book.php
index 8c8b9b8..c0a5baa 100644
--- a/book.php
+++ b/book.php
@@ -118,7 +118,7 @@ class Book extends Base {
         return $this->authors;
     }
     
-    public function getFilterString () {
+    public static function getFilterString () {
         $filter = getURLParam ("tag", NULL);
         if (empty ($filter)) return "";