diff --git a/src/calibre/db/backend.py b/src/calibre/db/backend.py
index 3aa4b09b50..c5a4bd5726 100644
--- a/src/calibre/db/backend.py
+++ b/src/calibre/db/backend.py
@@ -111,12 +111,12 @@ class DBPrefs(dict):  # {{{
         if not self.disable_setting:
             raw = self.to_raw(val)
             with self.db.conn:
-                dbraw = self.db.execute('SELECT val FROM preferences WHERE key=?', (key,)).fetchone()
-                if dbraw != (raw,):
+                dbraw = self.db.execute('SELECT id,val FROM preferences WHERE key=?', (key,)).fetchone()
+                if dbraw is None or dbraw[1] != 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))
+                        self.db.execute('UPDATE preferences SET val=? WHERE id=?', (raw, dbraw[0]))
                     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 9c764049f9..eefc682372 100644
--- a/src/calibre/db/tests/writing.py
+++ b/src/calibre/db/tests/writing.py
@@ -695,18 +695,20 @@ class WritingTest(BaseTest):
     def test_preferences(self):  # {{{
         ' Test getting and setting of preferences, especially with mutable objects '
         cache = self.init_cache()
+        changes = []
+        cache.backend.conn.setupdatehook(lambda typ, dbname, tblname, rowid: changes.append(rowid))
         prefs = cache.backend.prefs
         prefs['test mutable'] =  [1, 2, 3]
+        self.assertEqual(len(changes), 1)
         a = prefs['test mutable']
         a.append(4)
         self.assertIn(4, prefs['test mutable'])
         prefs['test mutable'] = a
+        self.assertEqual(len(changes), 2)
         prefs.load_from_db()
         self.assertIn(4, prefs['test mutable'])
-        changes = []
-        cache.backend.conn.setupdatehook(lambda typ, dbname, tblname, rowid: changes.append(rowid))
-        prefs['test mutable'] = {k:k for k in range(4)}
-        self.assertEqual(len(changes), 1)
-        prefs['test mutable'] = {k:k for k in range(4)}
-        self.assertEqual(len(changes), 1, 'The database was written to despite there being no change in value')
+        prefs['test mutable'] = {k:k for k in range(10)}
+        self.assertEqual(len(changes), 3)
+        prefs['test mutable'] = {k:k for k in reversed(range(10))}
+        self.assertEqual(len(changes), 3, 'The database was written to despite there being no change in value')
     # }}}