diff --git a/CHANGELOG.md b/CHANGELOG.md
index 2b4340c1..f52cf07e 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -7,6 +7,11 @@ project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
 
 ## [Unreleased]
 
+### Added
+
+- The `dotenv_path` argument of `set_key` and `unset_key` now has a type of `Union[str,
+  os.PathLike]` instead of just `os.PathLike` (#347 by [@bbc2]).
+
 ### Changed
 
 - Require Python 3.5 or a later version.  Python 2 and 3.4 are no longer supported. (#341
diff --git a/requirements.txt b/requirements.txt
index 952bfdce..39302b21 100644
--- a/requirements.txt
+++ b/requirements.txt
@@ -7,6 +7,7 @@ pytest-cov
 pytest>=3.9
 sh>=1.09
 tox
+types-mock
 wheel
 twine
 portray
diff --git a/setup.cfg b/setup.cfg
index 9afbc4b3..2723d8a2 100644
--- a/setup.cfg
+++ b/setup.cfg
@@ -14,6 +14,7 @@ max-line-length = 120
 exclude = .tox,.git,docs,venv,.venv
 
 [mypy]
+check_untyped_defs = true
 ignore_missing_imports = true
 
 [metadata]
diff --git a/src/dotenv/main.py b/src/dotenv/main.py
index 6b29fc90..d550f6f8 100644
--- a/src/dotenv/main.py
+++ b/src/dotenv/main.py
@@ -114,7 +114,7 @@ def get_key(dotenv_path: Union[str, _PathLike], key_to_get: str) -> Optional[str
 
 
 @contextmanager
-def rewrite(path: _PathLike) -> Iterator[Tuple[IO[str], IO[str]]]:
+def rewrite(path: Union[str, _PathLike]) -> Iterator[Tuple[IO[str], IO[str]]]:
     try:
         if not os.path.isfile(path):
             with io.open(path, "w+") as source:
@@ -131,7 +131,7 @@ def rewrite(path: _PathLike) -> Iterator[Tuple[IO[str], IO[str]]]:
 
 
 def set_key(
-    dotenv_path: _PathLike,
+    dotenv_path: Union[str, _PathLike],
     key_to_set: str,
     value_to_set: str,
     quote_mode: str = "always",
@@ -175,7 +175,7 @@ def set_key(
 
 
 def unset_key(
-    dotenv_path: _PathLike,
+    dotenv_path: Union[str, _PathLike],
     key_to_unset: str,
     quote_mode: str = "always",
 ) -> Tuple[Optional[bool], str]:
diff --git a/tox.ini b/tox.ini
index 7c2b4f9d..2cd63024 100644
--- a/tox.ini
+++ b/tox.ini
@@ -24,7 +24,8 @@ commands = coverage run --parallel -m pytest {posargs}
 skip_install = true
 deps =
   flake8
-  mypy<0.900
+  mypy
+  types-mock
 commands =
   flake8 src tests
   mypy --python-version=3.9 src tests