diff --git a/externs/topojson.js b/externs/topojson.js
index 3a606509a9..111f966ad7 100644
--- a/externs/topojson.js
+++ b/externs/topojson.js
@@ -75,6 +75,11 @@ TopoJSONGeometry.prototype.type;
 TopoJSONGeometry.prototype.id;
 
 
+/**
+ * @type {Object.<string, *>|undefined}
+ */
+TopoJSONGeometry.prototype.properties;
+
 
 /**
  * @constructor