diff --git a/miplearn/features/sample.py b/miplearn/features/sample.py
index 6407275..aa644e6 100644
--- a/miplearn/features/sample.py
+++ b/miplearn/features/sample.py
@@ -170,12 +170,16 @@ class Hdf5Sample(Sample):
 
     @overrides
     def get_bytes(self, key: str) -> Optional[bytes]:
+        if key not in self.file:
+            return None
         ds = self.file[key]
         assert len(ds.shape) == 1
         return ds[()].tobytes()
 
     @overrides
     def get_scalar(self, key: str) -> Optional[Any]:
+        if key not in self.file:
+            return None
         ds = self.file[key]
         assert len(ds.shape) == 0
         if h5py.check_string_dtype(ds.dtype):
@@ -185,6 +189,8 @@ class Hdf5Sample(Sample):
 
     @overrides
     def get_vector(self, key: str) -> Optional[Any]:
+        if key not in self.file:
+            return None
         ds = self.file[key]
         assert len(ds.shape) == 1
         print(ds.dtype)
@@ -197,6 +203,8 @@ class Hdf5Sample(Sample):
 
     @overrides
     def get_vector_list(self, key: str) -> Optional[Any]:
+        if key not in self.file:
+            return None
         ds = self.file[key]
         lens = ds.attrs["lengths"]
         if h5py.check_string_dtype(ds.dtype):
diff --git a/tests/features/test_sample.py b/tests/features/test_sample.py
index 0b57ef9..41452c2 100644
--- a/tests/features/test_sample.py
+++ b/tests/features/test_sample.py
@@ -39,6 +39,11 @@ def _test_sample(sample: Sample) -> None:
     # Bytes
     _assert_roundtrip_bytes(sample, b"\x00\x01\x02\x03\x04\x05")
 
+    assert sample.get_scalar("unknown-key") is None
+    assert sample.get_vector("unknown-key") is None
+    assert sample.get_vector_list("unknown-key") is None
+    assert sample.get_bytes("unknown-key") is None
+
 
 def _assert_roundtrip_bytes(sample: Sample, expected: Any) -> None:
     sample.put_bytes("key", expected)