diff --git a/src/calibre/db/legacy.py b/src/calibre/db/legacy.py
index ebfe6b7199..359d368ab6 100644
--- a/src/calibre/db/legacy.py
+++ b/src/calibre/db/legacy.py
@@ -7,6 +7,7 @@ __copyright__ = '2013, Kovid Goyal <kovid at kovidgoyal.net>'
 
 import os, traceback, weakref
 from polyglot.builtins import iteritems, zip
+from collections.abc import MutableMapping
 
 from calibre import force_unicode, isbytestring
 from calibre.constants import preferred_encoding
@@ -52,7 +53,7 @@ def set_global_state(db):
     set_saved_searches(db, 'saved_searches')
 
 
-class ThreadSafePrefs:
+class ThreadSafePrefs(MutableMapping):
 
     def __init__(self, db):
         self.db = weakref.ref(db)
diff --git a/src/calibre/db/tests/legacy.py b/src/calibre/db/tests/legacy.py
index bf71f715e0..af95b0ea76 100644
--- a/src/calibre/db/tests/legacy.py
+++ b/src/calibre/db/tests/legacy.py
@@ -189,7 +189,7 @@ class LegacyTest(BaseTest):
         db = self.init_old()
         newstag = ndb.new_api.get_item_id('tags', 'news')
 
-        self.assertEqual(dict(db.prefs), ndb.prefs.copy())
+        self.assertEqual(dict(db.prefs), dict(ndb.prefs))
 
         for meth, args in iteritems({
             'find_identical_books': [(Metadata('title one', ['author one']),), (Metadata('unknown'),), (Metadata('xxxx'),)],