From e111569c99f1b16f9a21b46235e4b02b36c3d1aa Mon Sep 17 00:00:00 2001
From: Kovid Goyal <kovid@kovidgoyal.net>
Date: Mon, 20 Jun 2022 14:12:12 +0530
Subject: [PATCH] ...

---
 src/calibre/ebooks/oeb/base.py | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/src/calibre/ebooks/oeb/base.py b/src/calibre/ebooks/oeb/base.py
index 8934a4d174..b798c0fc13 100644
--- a/src/calibre/ebooks/oeb/base.py
+++ b/src/calibre/ebooks/oeb/base.py
@@ -1903,7 +1903,10 @@ class OEBBook:
         return
 
     def _to_ncx(self):
-        lang = str(self.metadata.language[0])
+        try:
+            lang = str(self.metadata.language[0])
+        except IndexError:
+            lang = 'en'
         lang = lang.replace('_', '-')
         ncx = etree.Element(NCX('ncx'),
             attrib={'version': '2005-1', XML('lang'): lang},