diff --git a/miplearn/features/sample.py b/miplearn/features/sample.py
index aa644e6..e75f42e 100644
--- a/miplearn/features/sample.py
+++ b/miplearn/features/sample.py
@@ -132,6 +132,8 @@ class MemorySample(Sample):
 
     @overrides
     def put_scalar(self, key: str, value: Scalar) -> None:
+        if value is None:
+            return
         self._assert_is_scalar(value)
         self._put(key, value)
 
@@ -220,6 +222,8 @@ class Hdf5Sample(Sample):
 
     @overrides
     def put_scalar(self, key: str, value: Any) -> None:
+        if value is None:
+            return
         self._assert_is_scalar(value)
         self._put(key, value)
 
diff --git a/tests/features/test_sample.py b/tests/features/test_sample.py
index 41452c2..e07355f 100644
--- a/tests/features/test_sample.py
+++ b/tests/features/test_sample.py
@@ -39,11 +39,16 @@ def _test_sample(sample: Sample) -> None:
     # Bytes
     _assert_roundtrip_bytes(sample, b"\x00\x01\x02\x03\x04\x05")
 
+    # Querying unknown keys should return None
     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
 
+    # Putting None should not modify HDF5 file
+    sample.put_scalar("key", None)
+    sample.put_vector("key", None)
+
 
 def _assert_roundtrip_bytes(sample: Sample, expected: Any) -> None:
     sample.put_bytes("key", expected)