diff --git a/src/ol/structs/rbush.js b/src/ol/structs/rbush.js
index 536b3338a9..59e43722de 100644
--- a/src/ol/structs/rbush.js
+++ b/src/ol/structs/rbush.js
@@ -20,6 +20,7 @@
 // THE SOFTWARE.
 
 // FIXME bulk inserts
+// FIXME is level argument needed to insert_?
 
 goog.provide('ol.structs.RBush');