diff --git a/waddrmgr/manager.go b/waddrmgr/manager.go
index 97a7d94..0faf52c 100644
--- a/waddrmgr/manager.go
+++ b/waddrmgr/manager.go
@@ -1391,7 +1391,9 @@ func (m *Manager) MarkUsed(address btcutil.Address) error {
 		return maybeConvertDbError(err)
 	}
 	// Clear caches which might have stale entries for used addresses
+	m.mtx.Lock()
 	delete(m.addrs, addrKey(addressID))
+	m.mtx.Unlock()
 	return nil
 }