diff --git a/src/calibre/ebooks/oeb/polish/css.py b/src/calibre/ebooks/oeb/polish/css.py
index 123365035a..39fa1063e8 100644
--- a/src/calibre/ebooks/oeb/polish/css.py
+++ b/src/calibre/ebooks/oeb/polish/css.py
@@ -106,7 +106,10 @@ def filter_used_rules(root, rules, log, pseudo_pat, cache):
 def remove_unused_css(container, report):
     from cssutils.css import CSSRule
     sheets = {name:container.parsed(name) for name, mt in container.mime_map.iteritems() if mt in OEB_STYLES}
+    namespaced_sheets = set()
     for sheet in sheets.itervalues():
+        if 'h' not in sheet.namespaces:
+            namespaced_sheets.add(sheet)
         sheet.namespaces['h'] = XHTML_NS
     style_rules = {name:tuple(sheet.cssRules.rulesOfType(CSSRule.STYLE_RULE)) for name, sheet in sheets.iteritems()}
 
@@ -121,12 +124,15 @@ def remove_unused_css(container, report):
         for style in root.xpath('//*[local-name()="style"]'):
             if style.get('type', 'text/css') == 'text/css' and style.text:
                 sheet = container.parse_css(style.text)
+                ns = 'h' not in sheet.namespaces
                 sheet.namespaces['h'] = XHTML_NS
                 rules = tuple(sheet.cssRules.rulesOfType(CSSRule.STYLE_RULE))
                 unused_rules = tuple(filter_used_rules(root, rules, container.log, pseudo_pat, cache))
                 if unused_rules:
                     num_of_removed_rules += len(unused_rules)
                     [sheet.cssRules.remove(r) for r in unused_rules]
+                    if ns:
+                        del sheet.namespaces['h']
                     style.text = force_unicode(sheet.cssText, 'utf-8')
                     pretty_script_or_style(container, style)
                     container.dirty(name)
@@ -137,6 +143,8 @@ def remove_unused_css(container, report):
                 style_rules[sname] = tuple(filter_used_rules(root, style_rules[sname], container.log, pseudo_pat, cache))
 
     for name, sheet in sheets.iteritems():
+        if sheet in namespaced_sheets:
+            del sheet.namespaces['h']
         unused_rules = style_rules[name]
         if unused_rules:
             num_of_removed_rules += len(unused_rules)