diff --git a/src/ol/format/format.js b/src/ol/format/format.js
index b426dbaebc..a7b119da9d 100644
--- a/src/ol/format/format.js
+++ b/src/ol/format/format.js
@@ -23,6 +23,12 @@ ol.format.Format = function() {
 };
 
 
+/**
+ * @return {Array.<string>} Extensions.
+ */
+ol.format.Format.prototype.getExtensions = goog.abstractMethod;
+
+
 /**
  * @return {ol.format.FormatType} Format.
  */