diff --git a/electrum/address_synchronizer.py b/electrum/address_synchronizer.py
index bb5863db8..32dfd0820 100644
--- a/electrum/address_synchronizer.py
+++ b/electrum/address_synchronizer.py
@@ -212,7 +212,7 @@ class AddressSynchronizer(Logger, EventListener):
             self.db.history[address] = []
             self.set_up_to_date(False)
         if self.synchronizer:
-            self.synchronizer.add(address)
+            self.synchronizer.add_address(address)
 
     def get_conflicting_transactions(self, tx_hash, tx: Transaction, include_self=False):
         """Returns a set of transaction hashes from the wallet history that are
diff --git a/electrum/scripts/watch_address.py b/electrum/scripts/watch_address.py
index 297ec2ea2..4403d44ec 100755
--- a/electrum/scripts/watch_address.py
+++ b/electrum/scripts/watch_address.py
@@ -32,12 +32,12 @@ class Notifier(SynchronizerBase):
     async def main(self):
         # resend existing subscriptions if we were restarted
         for addr in self.watched_addresses:
-            await self._add_address(addr)
+            self.add_address(addr)
         # main loop
         while True:
             addr = await self.watch_queue.get()
             self.watched_addresses.add(addr)
-            await self._add_address(addr)
+            self.add_address(addr)
 
     async def _on_address_status(self, addr, status):
         print_msg(f"addr {addr}, status {status}")
diff --git a/electrum/synchronizer.py b/electrum/synchronizer.py
index 1e8ee4f69..2c434e7ba 100644
--- a/electrum/synchronizer.py
+++ b/electrum/synchronizer.py
@@ -83,15 +83,17 @@ class SynchronizerBase(NetworkJobOnDefaultServer):
             # we are being cancelled now
             self.session.unsubscribe(self.status_queue)
 
-    def add(self, addr):
-        asyncio.run_coroutine_threadsafe(self._add_address(addr), self.asyncio_loop)
-
-    async def _add_address(self, addr: str):
-        # note: this method is async as add_queue.put_nowait is not thread-safe.
+    def add_address(self, addr: str) -> None:
+        """Add an address to subscribe to.
+        Thread-safe. When the method returns, the Synchronizer (e.g. is_up_to_date) will know about the address.
+        """
         if not is_address(addr): raise ValueError(f"invalid bitcoin address {addr}")
         if addr in self.requested_addrs: return
         self.requested_addrs.add(addr)
-        self.add_queue.put_nowait(addr)
+        async def enqueue():
+            # note: this method is async as add_queue.put_nowait is not thread-safe.
+            self.add_queue.put_nowait(addr)
+        asyncio.run_coroutine_threadsafe(enqueue(), self.asyncio_loop)
 
     async def _on_address_status(self, addr, status):
         """Handle the change of the status of an address."""
@@ -245,7 +247,7 @@ class Synchronizer(SynchronizerBase):
             await self._request_missing_txs(history, allow_server_not_finding_tx=True)
         # add addresses to bootstrap
         for addr in random_shuffled_copy(self.adb.get_addresses()):
-            await self._add_address(addr)
+            self.add_address(addr)
         # main loop
         while True:
             await asyncio.sleep(0.1)
@@ -271,12 +273,12 @@ class Notifier(SynchronizerBase):
     async def main(self):
         # resend existing subscriptions if we were restarted
         for addr in self.watched_addresses:
-            await self._add_address(addr)
+            self.add_address(addr)
         # main loop
         while True:
             addr, url = await self._start_watching_queue.get()
             self.watched_addresses[addr].append(url)
-            await self._add_address(addr)
+            self.add_address(addr)
 
     async def start_watching_addr(self, addr: str, url: str):
         await self._start_watching_queue.put((addr, url))