diff --git a/src/calibre/utils/icu.py b/src/calibre/utils/icu.py
index f1f94dc175..66ee8fd59f 100644
--- a/src/calibre/utils/icu.py
+++ b/src/calibre/utils/icu.py
@@ -82,6 +82,17 @@ def icu_sort_key(collator, obj):
             obj = obj.replace(b'\0', b'')
         return _secondary_collator.sort_key(obj)
 
+def icu_change_case(upper, locale, obj):
+    func = _icu.upper if upper else _icu.lower
+    try:
+        return func(locale, obj)
+    except TypeError:
+        if isinstance(obj, unicode):
+            obj = obj.replace(u'\0', u'')
+        else:
+            obj = obj.replace(b'\0', b'')
+        return func(locale, obj)
+
 def py_find(pattern, source):
     pos = source.find(pattern)
     if pos > -1:
@@ -163,10 +174,10 @@ case_sensitive_sort_key = py_case_sensitive_sort_key if _icu_not_ok else \
 case_sensitive_strcmp = cmp if _icu_not_ok else icu_case_sensitive_strcmp
 
 upper = (lambda s: s.upper()) if _icu_not_ok else \
-    partial(_icu.upper, get_locale())
+    partial(icu_change_case, True, get_locale())
 
 lower = (lambda s: s.lower()) if _icu_not_ok else \
-    partial(_icu.lower, get_locale())
+    partial(icu_change_case, False, get_locale())
 
 title_case = (lambda s: s.title()) if _icu_not_ok else \
     partial(_icu.title, get_locale())