diff --git a/src/order_matching/matching_engine.py b/src/order_matching/matching_engine.py
index 32f1d11..f9104ae 100644
--- a/src/order_matching/matching_engine.py
+++ b/src/order_matching/matching_engine.py
@@ -46,7 +46,7 @@ def __init__(self, seed: int | None = None) -> None:
         self._faker = get_faker(seed=seed)
         self._queue = Orders()
         self.unprocessed_orders = OrderBook()
-        self._timestamp: datetime
+        self._timestamp: datetime | None = None
 
     def match(self, timestamp: datetime, orders: Orders | None = None) -> ExecutedTrades:
         """Match incoming orders in price-time priority.
@@ -73,7 +73,9 @@ def match(self, timestamp: datetime, orders: Orders | None = None) -> ExecutedTr
 
     def _get_expired_orders(self) -> Orders:
         orders: list[Order] = list()
-        for timestamp in filter(lambda t: t <= self._timestamp, self.unprocessed_orders.orders_by_expiration.keys()):
+        assert self._timestamp is not None
+        current_timestamp = self._timestamp
+        for timestamp in filter(lambda t: t <= current_timestamp, self.unprocessed_orders.orders_by_expiration.keys()):
             orders.extend(self.unprocessed_orders.orders_by_expiration[timestamp])
         for order in orders:
             order.status = Status.CANCEL
@@ -116,6 +118,7 @@ def _execute_trades_for_one_price(self, incoming_order: Order, price: float) ->
         return ExecutedTrades(trades=trades)
 
     def _execute_trade(self, incoming_order: Order, book_order: Order) -> Trade:
+        assert self._timestamp is not None
         trade = Trade(
             side=incoming_order.side,
             price=book_order.price,
diff --git a/src/order_matching/orders.py b/src/order_matching/orders.py
index d37eb53..156b454 100644
--- a/src/order_matching/orders.py
+++ b/src/order_matching/orders.py
@@ -1,7 +1,7 @@
 from __future__ import annotations
 
 from dataclasses import asdict
-from typing import Generator, Iterator, Sequence, cast
+from typing import Iterator, Sequence, cast
 
 import pandas as pd
 from pandera.typing.pandas import DataFrame
@@ -102,8 +102,8 @@ def __eq__(self, other: object) -> bool:
     def __iter__(self) -> Iterator[Order]:
         return iter(self.orders)
 
-    def __next__(self) -> Generator[Order, None, None]:
-        yield next(self.__iter__())
+    def __next__(self) -> Order:
+        return next(self.__iter__())
 
     def __len__(self) -> int:
         return len(self.orders)