diff --git a/course/lib.php b/course/lib.php
index 766c775adf3..3384bf3db41 100644
--- a/course/lib.php
+++ b/course/lib.php
@@ -2091,6 +2091,11 @@ function get_course_category_tree($id = 0, $depth = 0) {
         return array($categories, $categoryids);
     }
 
+    if (empty($categoryids)) {
+        // No categories available (probably all hidden).
+        return array();
+    }
+
     // The depth is 0 this function has just been called so we can finish it off
 
     list($ccselect, $ccjoin) = context_instance_preload_sql('c.id', CONTEXT_COURSE, 'ctx');