diff --git a/thumbnail.php b/thumbnail.php
index a5a2094..1740ee4 100644
--- a/thumbnail.php
+++ b/thumbnail.php
@@ -1,6 +1,6 @@
 <?php
     require("classes.php");
-    if (isset($_GET["grid"]))
+    if (isset($_GET["grid"]) && preg_match("/^[1-9.]{81}$/", $_GET["grid"]))
         $currentGrid = $_GET["grid"];
     else
         $currentGrid = ".528.3....4.9.1...39.562......73.129...1.64.7...42.3656.13.5...28.6.4...4.5287...";