diff --git a/src/ol/source/imagetilesource.js b/src/ol/source/imagetilesource.js
index 87e8ad0f51..8769cf71cf 100644
--- a/src/ol/source/imagetilesource.js
+++ b/src/ol/source/imagetilesource.js
@@ -112,6 +112,9 @@ ol.source.TileImage.prototype.getTile = function(z, x, y, projection) {
  * @param {ol.TileUrlFunctionType} tileUrlFunction Tile URL function.
  */
 ol.source.TileImage.prototype.setTileUrlFunction = function(tileUrlFunction) {
+  // FIXME It should be possible to be more intelligent and avoid clearing the
+  // FIXME cache.  The tile URL function would need to be incorporated into the
+  // FIXME cache key somehow.
   this.tileCache_.clear();
   this.tileUrlFunction = tileUrlFunction;
   this.dispatchChangeEvent();