diff --git a/can/io/asc.py b/can/io/asc.py
index c8a2aada3..1ad3acef6 100644
--- a/can/io/asc.py
+++ b/can/io/asc.py
@@ -6,6 +6,9 @@
     - under `test/data/logfile.asc`
 """
 
+from typing import cast, Any, Generator, IO, List, Optional, Tuple, Union
+from can import typechecking
+
 from datetime import datetime
 import time
 import logging
@@ -32,7 +35,11 @@ class ASCReader(BaseIOHandler):
     TODO: turn relative timestamps back to absolute form
     """
 
-    def __init__(self, file, base="hex"):
+    def __init__(
+        self,
+        file: Union[typechecking.FileLike, typechecking.StringPathLike],
+        base: str = "hex",
+    ) -> None:
         """
         :param file: a path-like object or as file-like object to read from
                      If this is a file-like object, is has to opened in text
@@ -42,10 +49,13 @@ def __init__(self, file, base="hex"):
                      this value will be overwritten. Default "hex".
         """
         super().__init__(file, mode="r")
+
+        if not self.file:
+            raise ValueError("The given file cannot be None")
         self.base = base
 
     @staticmethod
-    def _extract_can_id(str_can_id, base):
+    def _extract_can_id(str_can_id: str, base: int) -> Tuple[int, bool]:
         if str_can_id[-1:].lower() == "x":
             is_extended = True
             can_id = int(str_can_id[0:-1], base)
@@ -55,13 +65,15 @@ def _extract_can_id(str_can_id, base):
         return can_id, is_extended
 
     @staticmethod
-    def _check_base(base):
+    def _check_base(base: str) -> int:
         if base not in ["hex", "dec"]:
             raise ValueError('base should be either "hex" or "dec"')
         return BASE_DEC if base == "dec" else BASE_HEX
 
-    def __iter__(self):
+    def __iter__(self) -> Generator[Message, None, None]:
         base = self._check_base(self.base)
+        # This is guaranteed to not be None since we raise ValueError in __init__
+        self.file = cast(IO[Any], self.file)
         for line in self.file:
             # logger.debug("ASCReader: parsing line: '%s'", line.splitlines()[0])
             if line.split(" ")[0] == "base":
@@ -194,7 +206,11 @@ class ASCWriter(BaseIOHandler, Listener):
     FORMAT_DATE = "%a %b %d %I:%M:%S.{} %p %Y"
     FORMAT_EVENT = "{timestamp: 9.6f} {message}\n"
 
-    def __init__(self, file, channel=1):
+    def __init__(
+        self,
+        file: Union[typechecking.FileLike, typechecking.StringPathLike],
+        channel: int = 1,
+    ) -> None:
         """
         :param file: a path-like object or as file-like object to write to
                      If this is a file-like object, is has to opened in text
@@ -203,6 +219,9 @@ def __init__(self, file, channel=1):
                         have a channel set
         """
         super().__init__(file, mode="w")
+        if not self.file:
+            raise ValueError("The given file cannot be None")
+
         self.channel = channel
 
         # write start of file header
@@ -213,24 +232,29 @@ def __init__(self, file, channel=1):
 
         # the last part is written with the timestamp of the first message
         self.header_written = False
-        self.last_timestamp = None
-        self.started = None
+        self.last_timestamp = 0.0
+        self.started = 0.0
 
-    def stop(self):
+    def stop(self) -> None:
+        # This is guaranteed to not be None since we raise ValueError in __init__
+        self.file = cast(IO[Any], self.file)
         if not self.file.closed:
             self.file.write("End TriggerBlock\n")
         super().stop()
 
-    def log_event(self, message, timestamp=None):
+    def log_event(self, message: str, timestamp: Optional[float] = None) -> None:
         """Add a message to the log file.
 
-        :param str message: an arbitrary message
-        :param float timestamp: the absolute timestamp of the event
+        :param message: an arbitrary message
+        :param timestamp: the absolute timestamp of the event
         """
 
         if not message:  # if empty or None
             logger.debug("ASCWriter: ignoring empty message")
             return
+        # This is guaranteed to not be None since we raise ValueError in __init__
+        self.file = cast(IO[Any], self.file)
+
         # this is the case for the very first message:
         if not self.header_written:
             self.last_timestamp = timestamp or 0.0
@@ -251,14 +275,14 @@ def log_event(self, message, timestamp=None):
         line = self.FORMAT_EVENT.format(timestamp=timestamp, message=message)
         self.file.write(line)
 
-    def on_message_received(self, msg):
+    def on_message_received(self, msg: Message) -> None:
 
         if msg.is_error_frame:
             self.log_event("{}  ErrorFrame".format(self.channel), msg.timestamp)
             return
         if msg.is_remote_frame:
             dtype = "r"
-            data = []
+            data: List[str] = []
         else:
             dtype = "d {}".format(msg.dlc)
             data = ["{:02X}".format(byte) for byte in msg.data]