diff --git a/container/dangerzone.py b/container/dangerzone.py
index f69529b..4996c23 100644
--- a/container/dangerzone.py
+++ b/container/dangerzone.py
@@ -30,7 +30,9 @@ TIMEOUT_PER_MB: float = 30  # (seconds)
 TIMEOUT_MIN: float = 60  # (seconds)
 
 
-async def read_stream(sr: asyncio.StreamReader, callback: Callable = None) -> bytes:
+async def read_stream(
+    sr: asyncio.StreamReader, callback: Optional[Callable] = None
+) -> bytes:
     """Consume a byte stream line-by-line.
 
     Read all lines in a stream until EOF. If a user has passed a callback, call it for
@@ -59,8 +61,8 @@ async def run_command(
     error_message: str,
     timeout_message: str,
     timeout: Optional[float],
-    stdout_callback: Callable = None,
-    stderr_callback: Callable = None,
+    stdout_callback: Optional[Callable] = None,
+    stderr_callback: Optional[Callable] = None,
 ) -> Tuple[bytes, bytes]:
     """Run a command and get its output.
 
diff --git a/dangerzone/args.py b/dangerzone/args.py
index 8deb01d..f273e65 100644
--- a/dangerzone/args.py
+++ b/dangerzone/args.py
@@ -105,4 +105,4 @@ def override_parser_and_check_suspicious_options(click_main: click.Command) -> N
         check_suspicious_options(args)
         return orig_parse_fn(ctx, args)
 
-    click_main.parse_args = custom_parse_fn  # type: ignore [assignment]
+    click_main.parse_args = custom_parse_fn  # type: ignore [method-assign]
diff --git a/dangerzone/document.py b/dangerzone/document.py
index 36145a9..9013713 100644
--- a/dangerzone/document.py
+++ b/dangerzone/document.py
@@ -33,8 +33,8 @@ class Document:
 
     def __init__(
         self,
-        input_filename: str = None,
-        output_filename: str = None,
+        input_filename: Optional[str] = None,
+        output_filename: Optional[str] = None,
         suffix: str = SAFE_EXTENSION,
         archive: bool = False,
     ) -> None:
diff --git a/dangerzone/gui/__init__.py b/dangerzone/gui/__init__.py
index ca7dee5..a40a7e4 100644
--- a/dangerzone/gui/__init__.py
+++ b/dangerzone/gui/__init__.py
@@ -55,7 +55,7 @@ class Application(QtWidgets.QApplication):
 
             return self.original_event(event)
 
-        self.event = monkeypatch_event  # type: ignore [assignment]
+        self.event = monkeypatch_event  # type: ignore [method-assign]
 
 
 @click.command()
diff --git a/dangerzone/gui/logic.py b/dangerzone/gui/logic.py
index cf0c062..654a2ac 100644
--- a/dangerzone/gui/logic.py
+++ b/dangerzone/gui/logic.py
@@ -6,7 +6,7 @@ import shlex
 import subprocess
 import typing
 from pathlib import Path
-from typing import Dict
+from typing import Dict, Optional
 
 from colorama import Fore
 
@@ -129,7 +129,7 @@ class Alert(QtWidgets.QDialog):
         message: str,
         ok_text: str = "Ok",
         has_cancel: bool = True,
-        extra_button_text: str = None,
+        extra_button_text: Optional[str] = None,
     ) -> None:
         super(Alert, self).__init__()
         self.dangerzone = dangerzone
diff --git a/dangerzone/gui/main_window.py b/dangerzone/gui/main_window.py
index 413779d..1f9abd8 100644
--- a/dangerzone/gui/main_window.py
+++ b/dangerzone/gui/main_window.py
@@ -665,7 +665,7 @@ class ConvertTask(QtCore.QObject):
         self,
         dangerzone: DangerzoneGui,
         document: Document,
-        ocr_lang: str = None,
+        ocr_lang: Optional[str] = None,
     ) -> None:
         super(ConvertTask, self).__init__()
         self.document = document
diff --git a/dangerzone/isolation_provider/dummy.py b/dangerzone/isolation_provider/dummy.py
index b0ec822..16b8967 100644
--- a/dangerzone/isolation_provider/dummy.py
+++ b/dangerzone/isolation_provider/dummy.py
@@ -28,7 +28,7 @@ class Dummy(IsolationProvider):
             )
 
     def install(self) -> bool:
-        pass
+        return True
 
     def _convert(
         self,
diff --git a/tests/test_cli.py b/tests/test_cli.py
index 24db954..76e8aa0 100644
--- a/tests/test_cli.py
+++ b/tests/test_cli.py
@@ -9,7 +9,7 @@ import sys
 import tempfile
 import traceback
 from pathlib import Path
-from typing import Sequence
+from typing import Optional, Sequence
 from unittest import mock
 
 import pytest
@@ -56,7 +56,9 @@ class CLIResult(Result):
             self.print_info()
             raise
 
-    def assert_failure(self, exit_code: int = None, message: str = None) -> None:
+    def assert_failure(
+        self, exit_code: Optional[int] = None, message: Optional[str] = None
+    ) -> None:
         """Assert that the command failed.
 
         By default, check that the command has returned with an exit code
@@ -111,7 +113,7 @@ class CLIResult(Result):
 
 class TestCli(TestBase):
     def run_cli(
-        self, args: Sequence[str] | str = (), tmp_path: Path = None
+        self, args: Sequence[str] | str = (), tmp_path: Optional[Path] = None
     ) -> CLIResult:
         """Run the CLI with the provided arguments.