diff --git a/electrum/address_synchronizer.py b/electrum/address_synchronizer.py
index 40a8a5e58..843f3679f 100644
--- a/electrum/address_synchronizer.py
+++ b/electrum/address_synchronizer.py
@@ -60,7 +60,7 @@ class AddressSynchronizer(Logger):
     inherited by wallet
     """
 
-    def __init__(self, db: JsonDB):
+    def __init__(self, db: 'JsonDB'):
         self.db = db
         self.network = None  # type: Network
         Logger.__init__(self)