diff --git a/src/calibre/ebooks/mobi/reader.py b/src/calibre/ebooks/mobi/reader.py
index 9c52a18691..f1b1b1ef63 100644
--- a/src/calibre/ebooks/mobi/reader.py
+++ b/src/calibre/ebooks/mobi/reader.py
@@ -18,6 +18,7 @@ from calibre import xml_entity_to_unicode, CurrentDir, entity_to_unicode, \
     replace_entities
 from calibre.utils.filenames import ascii_filename
 from calibre.utils.date import parse_date
+from calibre.utils.cleantext import clean_ascii_chars
 from calibre.ptempfile import TemporaryDirectory
 from calibre.ebooks import DRMError
 from calibre.ebooks.chardet import ENCODING_PATS
@@ -323,6 +324,7 @@ class MobiReader(object):
         self.cleanup_html()
 
         self.log.debug('Parsing HTML...')
+        self.processed_html = clean_ascii_chars(self.processed_html)
         try:
             root = html.fromstring(self.processed_html)
             if len(root.xpath('//html')) > 5:
diff --git a/src/calibre/library/database2.py b/src/calibre/library/database2.py
index 8e90fe77bd..1762fd16d2 100644
--- a/src/calibre/library/database2.py
+++ b/src/calibre/library/database2.py
@@ -1196,7 +1196,8 @@ class LibraryDatabase2(LibraryDatabase, SchemaUpgrade, CustomColumns):
             else:
                 new_cats['.'.join(comps)] = user_cats[k]
         try:
-            self.prefs.set('user_categories', new_cats)
+            if new_cats != user_cats:
+                self.prefs.set('user_categories', new_cats)
         except:
             pass
         return new_cats
diff --git a/src/calibre/library/prefs.py b/src/calibre/library/prefs.py
index 233c717897..4ef1dcb35a 100644
--- a/src/calibre/library/prefs.py
+++ b/src/calibre/library/prefs.py
@@ -49,8 +49,7 @@ class DBPrefs(dict):
         if self.disable_setting:
             return
         raw = self.to_raw(val)
-        self.db.conn.execute('DELETE FROM preferences WHERE key=?', (key,))
-        self.db.conn.execute('INSERT INTO preferences (key,val) VALUES (?,?)', (key,
+        self.db.conn.execute('INSERT OR REPLACE INTO preferences (key,val) VALUES (?,?)', (key,
             raw))
         self.db.conn.commit()
         dict.__setitem__(self, key, val)