diff --git a/src/calibre/utils/icu.py b/src/calibre/utils/icu.py
index c1221f0614..e062f9ef00 100644
--- a/src/calibre/utils/icu.py
+++ b/src/calibre/utils/icu.py
@@ -12,7 +12,7 @@ import sys
 is_narrow_build = sys.maxunicode < 0x10ffff
 
 # Setup code {{{
-import sys
+import codecs
 
 from calibre.constants import plugins
 from calibre.utils.config_base import tweaks
@@ -30,25 +30,26 @@ del err
 icu_unicode_version = getattr(_icu, 'unicode_version', None)
 _nmodes = {m:getattr(_icu, 'UNORM_'+m, None) for m in ('NFC', 'NFD', 'NFKC', 'NFKD', 'NONE', 'DEFAULT', 'FCD')}
 
-ascii_encodings = {b'ansi_x3.4-1968', b'ascii'}
+# Ensure that the python internal filesystem and default encodings are not ASCII
+def is_ascii(name):
+    try:
+        return codecs.lookup(name).name == b'ascii'
+    except (TypeError, LookupError):
+        return True
 try:
-    senc = sys.getdefaultencoding()
-    if not senc or senc.lower() in ascii_encodings:
+    if is_ascii(sys.getdefaultencoding()):
         _icu.set_default_encoding(b'utf-8')
-    del senc
 except:
     import traceback
     traceback.print_exc()
 
 try:
-    fenc = sys.getfilesystemencoding()
-    if not fenc or fenc.lower() in ascii_encodings:
+    if is_ascii(sys.getfilesystemencoding()):
         _icu.set_filesystem_encoding(b'utf-8')
-    del fenc
 except:
     import traceback
     traceback.print_exc()
-del ascii_encodings
+del is_ascii
 
 def collator():
     global _collator, _locale