diff --git a/src/calibre/ebooks/html/input.py b/src/calibre/ebooks/html/input.py
index 326afc00c0..413db1cc0b 100644
--- a/src/calibre/ebooks/html/input.py
+++ b/src/calibre/ebooks/html/input.py
@@ -296,8 +296,8 @@ class HTMLInput(InputFormatPlugin):
             return self._is_case_sensitive
         if not path or not os.path.exists(path):
             return islinux or isfreebsd
-        self._is_case_sensitive = os.path.exists(path.lower()) \
-                and os.path.exists(path.upper())
+        self._is_case_sensitive = not (os.path.exists(path.lower()) \
+                and os.path.exists(path.upper()))
         return self._is_case_sensitive
 
     def create_oebbook(self, htmlpath, basedir, opts, log, mi):