diff --git a/base.json b/base.json
index 142140ce81..e4d98e1efa 100644
--- a/base.json
+++ b/base.json
@@ -21,6 +21,7 @@
 
   "externs": [
     "externs/bingmaps.js",
+    "externs/proj4js.js",
     "externs/tilejson.js",
     "externs/webgl-debug.js"
   ],