diff --git a/src/calibre/ebooks/oeb/transforms/flatcss.py b/src/calibre/ebooks/oeb/transforms/flatcss.py
index 2bd300beab..b736458a87 100644
--- a/src/calibre/ebooks/oeb/transforms/flatcss.py
+++ b/src/calibre/ebooks/oeb/transforms/flatcss.py
@@ -474,7 +474,7 @@ class CSSFlattener(object):
                 # lower() because otherwise if the document uses the same class
                 # name with different case, both cases will apply, leading
                 # to incorrect results.
-                klass = ascii_text(STRIPNUM.sub('', classes.split()[0])).lower()
+                klass = ascii_text(STRIPNUM.sub('', classes.split()[0])).lower().strip().replace(' ', '_')
                 if css in styles:
                     match = styles[css]
                 else: