diff --git a/miplearn/features/sample.py b/miplearn/features/sample.py
index 2c0c8f7..5503343 100644
--- a/miplearn/features/sample.py
+++ b/miplearn/features/sample.py
@@ -11,6 +11,7 @@ import numpy as np
 from h5py import Dataset
 from overrides import overrides
 
+Bytes = Union[bytes, bytearray]
 Scalar = Union[None, bool, str, int, float]
 Vector = Union[
     None,
@@ -37,11 +38,11 @@ class Sample(ABC):
     """Abstract dictionary-like class that stores training data."""
 
     @abstractmethod
-    def get_bytes(self, key: str) -> Optional[bytes]:
+    def get_bytes(self, key: str) -> Optional[Bytes]:
         pass
 
     @abstractmethod
-    def put_bytes(self, key: str, value: bytes) -> None:
+    def put_bytes(self, key: str, value: Bytes) -> None:
         pass
 
     @abstractmethod
@@ -115,7 +116,7 @@ class MemorySample(Sample):
         self._data: Dict[str, Any] = data
 
     @overrides
-    def get_bytes(self, key: str) -> Optional[bytes]:
+    def get_bytes(self, key: str) -> Optional[Bytes]:
         return self._get(key)
 
     @overrides
@@ -131,8 +132,10 @@ class MemorySample(Sample):
         return self._get(key)
 
     @overrides
-    def put_bytes(self, key: str, value: bytes) -> None:
-        assert isinstance(value, bytes), f"bytes expected; found: {value}"
+    def put_bytes(self, key: str, value: Bytes) -> None:
+        assert isinstance(
+            value, (bytes, bytearray)
+        ), f"bytes expected; found: {value}"  # type: ignore
         self._put(key, value)
 
     @overrides
@@ -176,7 +179,7 @@ class Hdf5Sample(Sample):
         self.file = h5py.File(filename, mode)
 
     @overrides
-    def get_bytes(self, key: str) -> Optional[bytes]:
+    def get_bytes(self, key: str) -> Optional[Bytes]:
         if key not in self.file:
             return None
         ds = self.file[key]
@@ -226,8 +229,10 @@ class Hdf5Sample(Sample):
         return _crop(padded, lens)
 
     @overrides
-    def put_bytes(self, key: str, value: bytes) -> None:
-        assert isinstance(value, bytes), f"bytes expected; found: {value}"
+    def put_bytes(self, key: str, value: Bytes) -> None:
+        assert isinstance(
+            value, (bytes, bytearray)
+        ), f"bytes expected; found: {value}"  # type: ignore
         self._put(key, np.frombuffer(value, dtype="uint8"))
 
     @overrides
diff --git a/tests/features/test_sample.py b/tests/features/test_sample.py
index 3051470..32b80bd 100644
--- a/tests/features/test_sample.py
+++ b/tests/features/test_sample.py
@@ -41,6 +41,11 @@ def _test_sample(sample: Sample) -> None:
 
     # Bytes
     _assert_roundtrip_bytes(sample, b"\x00\x01\x02\x03\x04\x05")
+    _assert_roundtrip_bytes(
+        sample,
+        bytearray(b"\x00\x01\x02\x03\x04\x05"),
+        check_type=False,
+    )
 
     # Querying unknown keys should return None
     assert sample.get_scalar("unknown-key") is None
@@ -53,12 +58,15 @@ def _test_sample(sample: Sample) -> None:
     sample.put_vector("key", None)
 
 
-def _assert_roundtrip_bytes(sample: Sample, expected: Any) -> None:
+def _assert_roundtrip_bytes(
+    sample: Sample, expected: Any, check_type: bool = False
+) -> None:
     sample.put_bytes("key", expected)
     actual = sample.get_bytes("key")
     assert actual == expected
     assert actual is not None
-    _assert_same_type(actual, expected)
+    if check_type:
+        _assert_same_type(actual, expected)
 
 
 def _assert_roundtrip_scalar(sample: Sample, expected: Any) -> None: