diff --git a/pybtc/connector/connector.py b/pybtc/connector/connector.py
index ee8a6d2..f70f528 100644
--- a/pybtc/connector/connector.py
+++ b/pybtc/connector/connector.py
@@ -378,7 +378,7 @@ class Connector:
                    not self.utxo.save_process and \
                    self.utxo.checkpoints:
                     self.utxo.deleted_last_block = block["height"]
-                    self.utxo.pending_deleted += self.utxo.deleted
+                    self.utxo.pending_deleted = self.utxo.pending_deleted.union(self.utxo.deleted)
                     self.utxo.deleted = set()
                     self.loop.create_task(self.utxo.save_utxo())