diff --git a/pymodbus/transport/transport.py b/pymodbus/transport/transport.py
index e6b09ea1f..1e978cd5d 100644
--- a/pymodbus/transport/transport.py
+++ b/pymodbus/transport/transport.py
@@ -266,9 +266,11 @@ async def transport_listen(self) -> bool:
             self.loop = asyncio.get_running_loop()
         self.is_closing = False
         try:
-            self.transport = await self.call_create()
-            if isinstance(self.transport, tuple):
-                self.transport = self.transport[0]
+            _transport = await self.call_create()
+            if isinstance(_transport, tuple):
+                self.transport = _transport[0]
+            else:
+                self.transport = _transport
         except OSError as exc:
             Log.warning("Failed to start server {}", exc)
             # self.transport_close(intern=True)
@@ -435,7 +437,7 @@ def transport_close(self, intern: bool = False, reconnect: bool = False) -> None
 
     def reset_delay(self) -> None:
         """Reset wait time before next reconnect to minimal period."""
-        self.reconnect_delay_current = self.comm_params.reconnect_delay
+        self.reconnect_delay_current = self.comm_params.reconnect_delay or 0
 
     def is_active(self) -> bool:
         """Return true if connected/listening."""
@@ -469,6 +471,12 @@ def handle_new_connection(self) -> ModbusProtocol:
 
     async def do_reconnect(self) -> None:
         """Handle reconnect as a task."""
+        if not (
+            self.comm_params.reconnect_delay and self.comm_params.reconnect_delay_max
+        ):
+            raise AssertionError(
+                "do_reconnect should not be called if reconnect_delay is None"
+            )
         try:
             self.reconnect_delay_current = self.comm_params.reconnect_delay
             while True:
@@ -519,7 +527,7 @@ def __init__(self, protocol: ModbusProtocol, listen: int | None = None) -> None:
         asyncio.DatagramTransport.__init__(self)
         asyncio.Transport.__init__(self)
         self.protocol: ModbusProtocol = protocol
-        self.other_modem: NullModem = None
+        self.other_modem: NullModem | None = None
         self.listen = listen
         self.manipulator: Callable[[bytes], list[bytes]] | None = None
         self._is_closing = False
@@ -605,6 +613,8 @@ def sendto(self, data: bytes, _addr: Any = None) -> None:
 
     def write(self, data: bytes) -> None:
         """Send data."""
+        if not self.other_modem:
+            raise AssertionError("Missing other_modem")
         if not self.manipulator:
             self.other_modem.protocol.data_received(data)
             return
diff --git a/pymodbus/transport/transport_serial.py b/pymodbus/transport/transport_serial.py
index 63c1affa5..b3ca34b38 100644
--- a/pymodbus/transport/transport_serial.py
+++ b/pymodbus/transport/transport_serial.py
@@ -20,7 +20,7 @@ def __init__(self, loop, protocol, *args, **kwargs) -> None:
         super().__init__()
         self.async_loop = loop
         self._protocol: asyncio.BaseProtocol = protocol
-        self.sync_serial = serial.serial_for_url(*args, **kwargs)
+        self.sync_serial: serial.Serial | None = serial.serial_for_url(*args, **kwargs)
         self._write_buffer: list[bytes] = []
         self.poll_task: asyncio.Task | None = None
         self._poll_wait_time = 0.0005
@@ -59,12 +59,12 @@ def close(self, exc: Exception | None = None) -> None:
     def write(self, data) -> None:
         """Write some data to the transport."""
         self._write_buffer.append(data)
-        if not self.poll_task:
+        if not self.poll_task and self.sync_serial:
             self.async_loop.add_writer(self.sync_serial.fileno(), self._write_ready)
 
     def flush(self) -> None:
         """Clear output buffer and stops any more data being written."""
-        if not self.poll_task:
+        if not self.poll_task and self.sync_serial:
             self.async_loop.remove_writer(self.sync_serial.fileno())
         self._write_buffer.clear()
 
diff --git a/pyproject.toml b/pyproject.toml
index 17cd7f42d..d7a1c60b3 100644
--- a/pyproject.toml
+++ b/pyproject.toml
@@ -187,7 +187,7 @@ overgeneral-exceptions = "builtins.Exception"
 bad-functions = "map,input"
 
 [tool.mypy]
-strict_optional = false
+strict_optional = true
 show_error_codes = true
 local_partial_types = true
 strict_equality = true