diff --git a/src/calibre/ebooks/oeb/polish/check/parsing.py b/src/calibre/ebooks/oeb/polish/check/parsing.py
index e27345dc2d..72e5894ebb 100644
--- a/src/calibre/ebooks/oeb/polish/check/parsing.py
+++ b/src/calibre/ebooks/oeb/polish/check/parsing.py
@@ -48,6 +48,7 @@ class XMLParseError(BaseError):
              ' "do the wrong thing".')
 
     def __init__(self, msg, *args, **kwargs):
+        msg = msg or ''
         BaseError.__init__(self, 'Parsing failed: ' + msg, *args, **kwargs)
         m = mismatch_pat.search(msg)
         if m is not None: