diff --git a/externs/proj4js.js b/externs/proj4js.js
index 0dd711eb63..ebdf9566da 100644
--- a/externs/proj4js.js
+++ b/externs/proj4js.js
@@ -70,6 +70,12 @@ Proj4js.Proj.prototype.title;
 Proj4js.Proj.prototype.units;
 
 
+/**
+ * @type {string}
+ */
+Proj4js.Proj.prototype.srsCode;
+
+
 /**
  * @nosideeffects
  * @param {Proj4js.Proj} source