diff --git a/HISTORY.md b/HISTORY.md
index d2800bc..4475be0 100644
--- a/HISTORY.md
+++ b/HISTORY.md
@@ -1,5 +1,9 @@
 # History
 
+## 0.9.1 (2022-07-12)
+
+* Return a `frozenset` for segments
+
 ## 0.9.0 (2022-07-07)
 
 * Add `segments` property to the `PanImg` model containing the unique values in the image as a tuple of `int`s.
diff --git a/panimg/models.py b/panimg/models.py
index 2024b68..e2318b7 100644
--- a/panimg/models.py
+++ b/panimg/models.py
@@ -4,7 +4,7 @@
 import shutil
 from enum import Enum
 from pathlib import Path
-from typing import Any, Dict, List, NamedTuple, Optional, Set, Tuple
+from typing import Any, Dict, FrozenSet, List, NamedTuple, Optional, Set, Tuple
 from uuid import UUID, uuid4
 
 import numpy as np
@@ -155,7 +155,7 @@ class PanImg:
     series_instance_uid: str = ""
     study_description: str = ""
     series_description: str = ""
-    segments: Optional[Tuple[int, ...]] = None
+    segments: Optional[FrozenSet[int]] = None
 
 
 @dataclass(frozen=True)
@@ -274,14 +274,14 @@ def add_value_range_meta_data(cls, image: Image):  # noqa: B902, N805
         return image
 
     @property
-    def segments(self) -> Optional[Tuple[int, ...]]:
+    def segments(self) -> Optional[FrozenSet[int]]:
         if self.image.GetPixelIDValue() not in MASK_TYPE_PIXEL_IDS:
             return None
 
         segments = np.unique(GetArrayViewFromImage(self.image))
 
         if len(segments) <= MAXIMUM_SEGMENTS_LENGTH:
-            return tuple(segments)
+            return frozenset(segments)
         else:
             return None
 
diff --git a/pyproject.toml b/pyproject.toml
index 06e8442..2c0fbd0 100644
--- a/pyproject.toml
+++ b/pyproject.toml
@@ -1,6 +1,6 @@
 [tool.poetry]
 name = "panimg"
-version = "0.9.0"
+version = "0.9.1"
 description = "Conversion of medical images to MHA and TIFF."
 license = "Apache-2.0"
 authors = ["James Meakin <panimg@jmsmkn.com>"]
diff --git a/tests/test_models.py b/tests/test_models.py
index 0778069..a94a14d 100644
--- a/tests/test_models.py
+++ b/tests/test_models.py
@@ -168,28 +168,30 @@ def test_sitk_image_value_range(
         (
             "image_min10_max10.mha",
             image_builders.image_builder_mhd,
-            (
-                -10,
-                -9,
-                -8,
-                -7,
-                -6,
-                -5,
-                -4,
-                -3,
-                -2,
-                -1,
-                0,
-                1,
-                2,
-                3,
-                4,
-                5,
-                6,
-                7,
-                8,
-                9,
-                10,
+            frozenset(
+                (
+                    -10,
+                    -9,
+                    -8,
+                    -7,
+                    -6,
+                    -5,
+                    -4,
+                    -3,
+                    -2,
+                    -1,
+                    0,
+                    1,
+                    2,
+                    3,
+                    4,
+                    5,
+                    6,
+                    7,
+                    8,
+                    9,
+                    10,
+                )
             ),
         ),
         (  # Too many values