diff --git a/lib/formslib.php b/lib/formslib.php
index acb5db31ae8..1291d312ba9 100644
--- a/lib/formslib.php
+++ b/lib/formslib.php
@@ -211,7 +211,9 @@ abstract class moodleform {
      * @return string form identifier.
      */
     protected function get_form_identifier() {
-        return get_class($this);
+        $class = get_class($this);
+
+        return preg_replace('/[^a-z0-9_]/i', '_', $class);
     }
 
     /**