diff --git a/src/calibre/ebooks/oeb/base.py b/src/calibre/ebooks/oeb/base.py
index e85098e293..2d2945c26a 100644
--- a/src/calibre/ebooks/oeb/base.py
+++ b/src/calibre/ebooks/oeb/base.py
@@ -1908,6 +1908,7 @@ class OEBBook(object):
 
     def _to_ncx(self):
         lang = unicode(self.metadata.language[0])
+        lang = lang.replace('_', '-')
         ncx = etree.Element(NCX('ncx'),
             attrib={'version': '2005-1', XML('lang'): lang},
             nsmap={None: NCX_NS})