diff --git a/src/ol/source/Tile.js b/src/ol/source/Tile.js
index 79c3e598f7..c2f53d1b0a 100644
--- a/src/ol/source/Tile.js
+++ b/src/ol/source/Tile.js
@@ -107,7 +107,7 @@ TileSource.prototype.canExpireCache = function() {
 
 /**
  * @param {module:ol/proj/Projection} projection Projection.
- * @param {Object.<string, module:ol/TileRange>} usedTiles Used tiles.
+ * @param {!Object.<string, module:ol/TileRange>} usedTiles Used tiles.
  */
 TileSource.prototype.expireCache = function(projection, usedTiles) {
   const tileCache = this.getTileCacheForProjection(projection);