diff --git a/classroom.pages.inc b/classroom.pages.inc
index 4a0c603..f14ae56 100644
--- a/classroom.pages.inc
+++ b/classroom.pages.inc
@@ -645,8 +645,8 @@ function _classroom_course_topic_form_validate($form, $form_state) {
   }
 
   // Check if the number is duplicated yet.
-  $qry = "SELECT COUNT(*) AS e FROM {classroom_topic} WHERE number = %d";
-  $conds = array($form['#post']['number']);
+  $qry = "SELECT COUNT(*) AS e FROM {classroom_topic} WHERE number = %d AND course_nid = %d";
+  $conds = array($form['#post']['number'], $form['#post']['course_nid']);
   if ($form['#post']['tid']) {
     $qry .= " AND tid <> %d";
     $conds[] = $form['#post']['tid'];