diff --git a/externs/proj4js.js b/externs/proj4js.js
index e4c4341234..1eab482820 100644
--- a/externs/proj4js.js
+++ b/externs/proj4js.js
@@ -65,6 +65,7 @@ Proj4js.Proj.prototype.units;
 
 
 /**
+ * @nosideeffects
  * @param {Proj4js.Proj} source
  * @param {Proj4js.Proj} dest
  * @param {Proj4js.Point|{x:number, y:number}} point