diff --git a/src/calibre/ebooks/html/input.py b/src/calibre/ebooks/html/input.py
index 35c0acc097..326afc00c0 100644
--- a/src/calibre/ebooks/html/input.py
+++ b/src/calibre/ebooks/html/input.py
@@ -292,7 +292,7 @@ class HTMLInput(InputFormatPlugin):
                 encoding=opts.input_encoding)
 
     def is_case_sensitive(self, path):
-        if self._is_case_sensitive is not None:
+        if getattr(self, '_is_case_sensitive', None) is not None:
             return self._is_case_sensitive
         if not path or not os.path.exists(path):
             return islinux or isfreebsd