diff --git a/src/calibre/db/backend.py b/src/calibre/db/backend.py
index 5077d1f1a7..3aa4b09b50 100644
--- a/src/calibre/db/backend.py
+++ b/src/calibre/db/backend.py
@@ -90,7 +90,9 @@ class DBPrefs(dict):  # {{{
         return json.loads(raw, object_hook=from_json)
 
     def to_raw(self, val):
-        return json.dumps(val, indent=2, default=to_json)
+        # sort_keys=True is required so that the serialization of dictionaries is
+        # not random, which is needed for the changed check in __setitem__
+        return json.dumps(val, indent=2, default=to_json, sort_keys=True)
 
     def has_setting(self, key):
         return key in self
@@ -110,8 +112,11 @@ class DBPrefs(dict):  # {{{
             raw = self.to_raw(val)
             with self.db.conn:
                 dbraw = self.db.execute('SELECT val FROM preferences WHERE key=?', (key,)).fetchone()
-                if dbraw != raw:
-                    self.db.execute('INSERT OR REPLACE INTO preferences (key,val) VALUES (?,?)', (key, raw))
+                if dbraw != (raw,):
+                    if dbraw is None:
+                        self.db.execute('INSERT INTO preferences (key,val) VALUES (?,?)', (key, raw))
+                    else:
+                        self.db.execute('UPDATE preferences SET val=? WHERE key=?', (raw, key))
                     dict.__setitem__(self, key, val)
 
     def set(self, key, val):
diff --git a/src/calibre/db/tests/writing.py b/src/calibre/db/tests/writing.py
index eca53bd86b..8430090586 100644
--- a/src/calibre/db/tests/writing.py
+++ b/src/calibre/db/tests/writing.py
@@ -7,7 +7,6 @@ __license__   = 'GPL v3'
 __copyright__ = '2013, Kovid Goyal <kovid at kovidgoyal.net>'
 __docformat__ = 'restructuredtext en'
 
-import os
 from collections import namedtuple
 from functools import partial
 from io import BytesIO
@@ -704,7 +703,10 @@ class WritingTest(BaseTest):
         prefs['test mutable'] = a
         prefs.load_from_db()
         self.assertIn(4, prefs['test mutable'])
-        before = os.stat(cache.backend.library_path)
-        prefs['test mutable'] = a
-        self.assertEqual(before, os.stat(cache.backend.library_path), 'The database was written to despite there being no change in value')
+        changes = []
+        cache.backend.conn.setupdatehook(lambda typ, dbname, tblname, rowid: changes.append(rowid))
+        prefs['test mutable'] = {str(k):k for k in range(4)}
+        self.assertEqual(len(changes), 1)
+        prefs['test mutable'] = {str(k):k for k in range(4)}
+        self.assertEqual(len(changes), 1, 'The database was written to despite there being no change in value')
     # }}}