diff --git a/src/calibre/ebooks/html.py b/src/calibre/ebooks/html.py
index f551121f94..f6659f9f51 100644
--- a/src/calibre/ebooks/html.py
+++ b/src/calibre/ebooks/html.py
@@ -836,7 +836,7 @@ class Processor(Parser):
                     classname = 'calibre_class_%d'%class_counter
                     class_counter += 1
                     cache[setting] = classname
-                cn = elem.get('class', '')
+                cn = elem.get('class', classname)
                 elem.set('class', cn)
             elem.attrib.pop('style')
         
diff --git a/src/calibre/ebooks/oeb/transforms/flatcss.py b/src/calibre/ebooks/oeb/transforms/flatcss.py
index 8a3057787d..28b72b04f3 100644
--- a/src/calibre/ebooks/oeb/transforms/flatcss.py
+++ b/src/calibre/ebooks/oeb/transforms/flatcss.py
@@ -207,7 +207,8 @@ class CSSFlattener(object):
             items = cssdict.items()
             items.sort()
             css = u';\n'.join(u'%s: %s' % (key, val) for key, val in items)
-            klass = STRIPNUM.sub('', node.get('class', 'calibre').split()[0])
+            classes = node.get('class', None) or 'calibre'
+            klass = STRIPNUM.sub('', classes.split()[0])
             if css in styles:
                 match = styles[css]
             else: