diff --git a/src/calibre/spell/dictionary.py b/src/calibre/spell/dictionary.py
index c0c891da27..839468ec94 100644
--- a/src/calibre/spell/dictionary.py
+++ b/src/calibre/spell/dictionary.py
@@ -101,7 +101,10 @@ def custom_dictionaries(reread=False):
     return _custom
 
 default_en_locale = 'en-US'
-ul = parse_lang_code(get_system_locale() or 'en-US')
+try:
+    ul = parse_lang_code(get_system_locale() or 'en-US')
+except ValueError:
+    ul = None
 if ul is not None and ul.langcode == 'eng' and ul.countrycode in 'GB BS BZ GH IE IN JM NZ TT'.split():
     default_en_locale = 'en-' + ul.countrycode
 default_preferred_locales = {'eng':default_en_locale, 'deu':'de-DE', 'spa':'es-ES', 'fra':'fr-FR'}