diff --git a/js/map.js b/js/map.js
index f051e00de..e6406a606 100644
--- a/js/map.js
+++ b/js/map.js
@@ -365,7 +365,7 @@ jspb.Map.prototype.serializeBinary = function(
       valueWriterFn.call(writer, 2, this.wrapEntry_(entry),
                          opt_valueWriterCallback);
     } else {
-      valueWriterFn_.call(writer, 2, entry.value);
+      valueWriterFn.call(writer, 2, entry.value);
     }
     writer.endSubMessage();
   }