diff --git a/src/calibre/ebooks/oeb/transforms/flatcss.py b/src/calibre/ebooks/oeb/transforms/flatcss.py
index 12fbd3b7f1..03410e1a65 100644
--- a/src/calibre/ebooks/oeb/transforms/flatcss.py
+++ b/src/calibre/ebooks/oeb/transforms/flatcss.py
@@ -18,7 +18,7 @@ from calibre import guess_type
 from calibre.ebooks.oeb.base import (XHTML, XHTML_NS, CSS_MIME, OEB_STYLES,
         namespace, barename, XPath)
 from calibre.ebooks.oeb.stylizer import Stylizer
-from calibre.utils.filenames import ascii_filename
+from calibre.utils.filenames import ascii_filename, ascii_text
 
 COLLAPSE = re.compile(r'[ \t\r\n\v]+')
 STRIPNUM = re.compile(r'[-0-9]+$')
@@ -437,7 +437,7 @@ class CSSFlattener(object):
                 items.sort()
                 css = u';\n'.join(u'%s: %s' % (key, val) for key, val in items)
                 classes = node.get('class', '').strip() or 'calibre'
-                klass = STRIPNUM.sub('', classes.split()[0].replace('_', ''))
+                klass = ascii_text(STRIPNUM.sub('', classes.split()[0].replace('_', '')))
                 if css in styles:
                     match = styles[css]
                 else: