diff --git a/src/calibre/gui2/viewer/javascript.py b/src/calibre/gui2/viewer/javascript.py
index ab8492dc6f..e092d55f14 100644
--- a/src/calibre/gui2/viewer/javascript.py
+++ b/src/calibre/gui2/viewer/javascript.py
@@ -73,6 +73,9 @@ class JavaScriptLoader(object):
             src = self.get(x)
             evaljs(src)
 
+        if not lang:
+            lang = 'en'
+
         def lang_name(l):
             l = l.lower()
             l = lang_as_iso639_1(l)