diff --git a/mod/forum/discuss.php b/mod/forum/discuss.php
index 3f1a2ca06b4..c4788b0abb0 100644
--- a/mod/forum/discuss.php
+++ b/mod/forum/discuss.php
@@ -10,7 +10,7 @@
     $mode   = optional_param('mode', 0, PARAM_INT);          // If set, changes the layout of the thread
     $move   = optional_param('move', 0, PARAM_INT);          // If set, moves this discussion to another forum
     $fromforum = optional_param('fromforum', 0, PARAM_INT);  // Needs to be set when we want to move a discussion.
-    $mark   = optional_param('mark', 0, PARAM_INT);          // Used for tracking read posts if user initiated.
+    $mark   = optional_param('mark', '', PARAM_ALPHA);       // Used for tracking read posts if user initiated.
     $postid = optional_param('postid', 0, PARAM_INT);        // Used for tracking read posts if user initiated.
 
     if (!$discussion = get_record("forum_discussions", "id", $d)) {