diff --git a/lib/browserid/views.js b/lib/browserid/views.js
index 623604f13a88ccd2b7a3356bec098d3c8739d663..9b7309a0a46c46cc75851f846f36d032c41a887f 100644
--- a/lib/browserid/views.js
+++ b/lib/browserid/views.js
@@ -30,7 +30,7 @@ function renderCachableView(req, res, template, options) {
     if (config.get('env') === 'production') {
       // res.setHeader('Cache-Control', 'public, max-age=' + (maxAge / 1000));
       res.setHeader('Last-Modified', stat.mtime.toUTCString());
-      res.setHeader('ETag', util.format('"%s-%s-%s"', stat.size, stat.mtime, req.locale));
+      res.setHeader('ETag', util.format('"%s-%s-%s"', stat.size, Number(stat.mtime), req.locale));
     } else {
       res.setHeader('Cache-Control', 'no-store');
     }