--- a/src/calibre/spell/dictionary.py	2014-05-01 11:38:04.051290252 -0400
+++ b/src/calibre/spell/dictionary.py	2014-05-01 11:37:55.061290088 -0400
@@ -52,7 +52,7 @@
     raw = raw or ''
     parts = raw.replace('_', '-').split('-')
     lc = canonicalize_lang(parts[0])
-    if lc is None:
+    if lc is None and raw != 'C':
         raise ValueError('Invalid language code: %r' % raw)
     cc = None
     if len(parts) > 1: