diff --git a/h5p.classes.php b/h5p.classes.php
index 61153df..29ce875 100644
--- a/h5p.classes.php
+++ b/h5p.classes.php
@@ -268,7 +268,7 @@ class H5PValidator {
 
   private $libraryOptional  = array(
     'author' => '/^.{1,255}$/',
-    'license' => '/^(cc-by|cc-by-sa|cc-by-nd|cc-by-nc|cc-by-nc-sa|cc-by-nc-nd|pd|cr|MIT)$/',
+    'license' => '/^(cc-by|cc-by-sa|cc-by-nd|cc-by-nc|cc-by-nc-sa|cc-by-nc-nd|pd|cr|MIT|GPL1|GPL2|GPL3)$/',
     'description' => '/^.{1,}$/',
     'dynamicDependencies' => array(
       'machineName' => '/^[\w0-9\-\.]{1,255}$/i',