diff --git a/README.md b/README.md
index 1d14727..5bd0ee1 100644
--- a/README.md
+++ b/README.md
@@ -7,7 +7,7 @@ shared/exclusive mode semantics.
 
 `MultiLock.locked` can be in one of three states:
 
-1. `None` - not locked;
+1. `MultiLockType.NONE` - not acquired;
 2. `MultiLockType.SHARED` - acquired one or more times in shared mode;
 3. `MultiLockType.EXCLUSIVE` - acquired one time in exclusive mode.
 
diff --git a/src/asyncio_multilock/__init__.py b/src/asyncio_multilock/__init__.py
index fa5cab6..665fd3f 100644
--- a/src/asyncio_multilock/__init__.py
+++ b/src/asyncio_multilock/__init__.py
@@ -12,8 +12,6 @@
     Optional,
 )
 
-__sentinel__ = object()
-
 
 class LockError(Exception): ...
 
@@ -25,7 +23,7 @@ class HandleUsedError(LockError): ...
 
 
 class MultiLockType(IntEnum):
-    # 0 not used so that type is always truthy.
+    NONE = 0
     SHARED = 1
     EXCLUSIVE = 2
 
@@ -35,6 +33,18 @@ def max(a: MultiLockType, b: MultiLockType) -> MultiLockType:
             return b
         return a
 
+    def excludes(self, other: MultiLockType) -> bool:
+        if self is MultiLockType.NONE:
+            return False
+
+        return MultiLockType.EXCLUSIVE in (self, other)
+
+    def includes(self, other: MultiLockType) -> bool:
+        if self is MultiLockType.NONE:
+            return True
+
+        return MultiLockType.EXCLUSIVE not in (self, other)
+
 
 class MultiLock:
     """Shared/exclusive mode lock.
@@ -55,20 +65,20 @@ class MultiLock:
     If not given they will be created and returned as appropriate.
     """
 
-    def __init__(self):
-        self._locked: Optional[MultiLockType] = __sentinel__  # type: ignore
+    def __init__(self) -> None:
+        self._locked: Optional[MultiLockType] = None
         self._acquire: Dict[Hashable, MultiLockType] = {}
         self._notify: Dict[Event, MultiLockType] = {}
 
     @property
-    def locked(self) -> Optional[MultiLockType]:
+    def locked(self) -> MultiLockType:
         """Lock state.
 
         None if not locked. Acquired lock type otherwise.
         """
-        if self._locked is __sentinel__:
+        if self._locked is None:
             self._locked = (
-                None
+                MultiLockType.NONE
                 if not self._acquire
                 else reduce(MultiLockType.max, self._acquire.values())
             )
@@ -90,7 +100,7 @@ def notify(
             raise EventUsedError(f"{id(self)}: event {id(event)} already in notify")
         self._notify[event] = type
         try:
-            if not self.locked or MultiLockType.EXCLUSIVE not in (self.locked, type):
+            if self.locked.includes(type):
                 event.set()
             yield event
         finally:
@@ -115,7 +125,7 @@ def acquire_nowait(
         :param handle: User-specified lock handle.
         :return: Handle if lock is acquired. None otherwise.
         """
-        if self.locked and MultiLockType.EXCLUSIVE in (self.locked, type):
+        if self.locked.excludes(type):
             return None
         if handle is None:
             handle = object()
@@ -124,7 +134,7 @@ def acquire_nowait(
                 f"{id(self)}: handle {id(handle)} already in acquired"
             )
         self._acquire[handle] = type
-        self._locked = __sentinel__  # type: ignore
+        self._locked = None
         return handle
 
     async def acquire(
@@ -151,9 +161,9 @@ async def acquire(
 
     def release(self, handle: Hashable) -> None:
         self._acquire.pop(handle, None)  # OK to release unknown handle.
-        self._locked = __sentinel__  # type: ignore
+        self._locked = None
         for handle, type in self._notify.items():
-            if not self.locked or MultiLockType.EXCLUSIVE not in (self.locked, type):
+            if self.locked.includes(type):
                 handle.set()
 
     @asynccontextmanager
diff --git a/tests/test_multilock.py b/tests/test_multilock.py
index cdf4141..c27671a 100644
--- a/tests/test_multilock.py
+++ b/tests/test_multilock.py
@@ -38,7 +38,9 @@ def test_acquire_nowait_fail_when_exclusive(type: MultiLockType) -> None:
     assert not lock.locked
 
 
-@mark.parametrize(["type"], [param(type) for type in MultiLockType])
+@mark.parametrize(
+    ["type"], [param(MultiLockType.SHARED), param(MultiLockType.EXCLUSIVE)]
+)
 def test_acquire_nowait_fail_exclusive_when_locked(type: MultiLockType) -> None:
     lock = MultiLock()
     acquired = lock.acquire_nowait(type)
@@ -106,7 +108,9 @@ async def test_acquire_wait_when_exclusive(type: MultiLockType) -> None:
 
 
 @mark.timeout(3)
-@mark.parametrize(["type"], [param(type) for type in MultiLockType])
+@mark.parametrize(
+    ["type"], [param(MultiLockType.SHARED), param(MultiLockType.EXCLUSIVE)]
+)
 async def test_acquire_wait_exclusive_when_locked(type: MultiLockType):
     lock = MultiLock()
     acquired = await lock.acquire(type)
@@ -169,7 +173,9 @@ async def test_notify_wait_when_exclusive(type: MultiLockType):
 
 
 @mark.timeout(3)
-@mark.parametrize(["type"], [param(type) for type in MultiLockType])
+@mark.parametrize(
+    ["type"], [param(MultiLockType.SHARED), param(MultiLockType.EXCLUSIVE)]
+)
 async def test_notify_wait_exclusive_when_locked(type: MultiLockType):
     lock = MultiLock()
     handle = lock.acquire_nowait(type)