diff --git a/pyproject.toml b/pyproject.toml
index aef43b5..deb70b1 100644
--- a/pyproject.toml
+++ b/pyproject.toml
@@ -25,10 +25,10 @@ dependencies = ["chess==1.11.2"]
 
 [project.optional-dependencies]
 dev = [
-    "pytest==9.0.3",
-    "pytest-cov>=6",
-    "ruff==0.14.4",
-    "mypy==1.18.2",
+    "pytest==9.1.1",
+    "pytest-cov>=7.1.0",
+    "ruff==0.16.4",
+    "mypy==2.3.1",
     "pre-commit>=4.6.2",
 ]
 
diff --git a/src/pychess/shared_tt.py b/src/pychess/shared_tt.py
index abc7a9c..4c5d8a9 100644
--- a/src/pychess/shared_tt.py
+++ b/src/pychess/shared_tt.py
@@ -25,6 +25,14 @@
 _U64 = (1 << 64) - 1
 
 
+def _buffer(shm: shared_memory.SharedMemory) -> memoryview:
+    """Return the segment's buffer, which is ``None`` only once it is closed."""
+    buf = shm.buf
+    if buf is None:  # pragma: no cover - a live segment always has a buffer
+        raise RuntimeError("shared memory segment is closed")
+    return buf
+
+
 def _pack_move(move: chess.Move | None) -> int:
     if move is None:
         return 0
@@ -74,7 +82,7 @@ def probe(
         self, key: int, depth: int, alpha: int, beta: int
     ) -> tuple[bool, int, chess.Move | None]:
         off = (key & self.mask) * ENTRY_SIZE
-        word0, data = _ENTRY.unpack_from(self.shm.buf, off)
+        word0, data = _ENTRY.unpack_from(_buffer(self.shm), off)
         if data == 0 or (word0 ^ data) != key:
             return False, 0, None
         move, score, e_depth, flag = _unpack_data(data)
@@ -89,11 +97,11 @@ def probe(
 
     def store(self, key: int, depth: int, score: int, flag: int, move: chess.Move | None) -> None:
         off = (key & self.mask) * ENTRY_SIZE
-        word0, old = _ENTRY.unpack_from(self.shm.buf, off)
+        word0, old = _ENTRY.unpack_from(_buffer(self.shm), off)
         if old and (word0 ^ old) == key and _unpack_data(old)[2] > depth:
             return  # keep the deeper entry for this position
         data = _pack_data(move, score, depth, flag)
-        _ENTRY.pack_into(self.shm.buf, off, (key ^ data) & _U64, data)
+        _ENTRY.pack_into(_buffer(self.shm), off, (key ^ data) & _U64, data)
 
     def close(self) -> None:
         with contextlib.suppress(Exception):
@@ -110,16 +118,16 @@ class SharedFlag:
     def __init__(self, name: str | None = None, create: bool = True) -> None:
         if create:
             self.shm = shared_memory.SharedMemory(create=True, size=1)
-            self.shm.buf[0] = 0
+            _buffer(self.shm)[0] = 0
         else:
             self.shm = shared_memory.SharedMemory(name=name)
         self.name = self.shm.name
 
     def set(self) -> None:
-        self.shm.buf[0] = 1
+        _buffer(self.shm)[0] = 1
 
     def is_set(self) -> bool:
-        return self.shm.buf[0] != 0
+        return _buffer(self.shm)[0] != 0
 
     def close(self) -> None:
         with contextlib.suppress(Exception):