diff --git a/src/ol/format/polylineformat.js b/src/ol/format/polylineformat.js
index a50cef525f..69c46a84e6 100644
--- a/src/ol/format/polylineformat.js
+++ b/src/ol/format/polylineformat.js
@@ -97,6 +97,7 @@ ol.format.Polyline.decodeDeltas = function(encoded, dimension, opt_factor) {
   var factor = goog.isDef(opt_factor) ? opt_factor : 1e5;
   var d;
 
+  /** @type {Array.<number>} */
   var lastNumbers = new Array(dimension);
   for (d = 0; d < dimension; ++d) {
     lastNumbers[d] = 0;