diff --git a/CHANGELOG.md b/CHANGELOG.md
index 98046ea..f3dd9d2 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -98,6 +98,8 @@
     compound fields by name, variable-length data, ...;
     `docs/known-issues.md`).
 - `File.mode`, `File.flush()` (a no-op), `Dataset.chunks`.
+- Boolean-mask keys (`ds[mask]`, `ds[mask] = v`), which h5py supports,
+  raise `NotImplementedError` (they raised `TypeError`).
 - Tests (`tests/test_edit.py`): each edit applied to two copies of a file,
   by h5py and by clawhdf5, and both read back through h5py after every
   edit, on files h5py writes with `libver` earliest, v114 and latest and on
diff --git a/crates/clawhdf5-py/README.md b/crates/clawhdf5-py/README.md
index 50b0d17..06eebec 100644
--- a/crates/clawhdf5-py/README.md
+++ b/crates/clawhdf5-py/README.md
@@ -43,8 +43,9 @@ with clawhdf5.File("data.h5", "r") as f:
   Other types raise `TypeError`.
 - Keys are h5py's: integers, slices with a positive step, `...`, one
   increasing list of integers, compound field names. Each maps onto a
-  hyperslab selection. `None`, negative steps and boolean masks are refused
-  with h5py's errors.
+  hyperslab selection. `None` and negative steps are refused
+  with h5py's errors; boolean masks (which h5py supports) raise
+  `NotImplementedError`, for reads and writes.
 - What is read from the file: a selection whose bounding box covers at
   most half the dataset decodes only the chunks (or contiguous rows) the box
   overlaps. The library decodes the whole dataset for a larger box
diff --git a/crates/clawhdf5-py/src/select.rs b/crates/clawhdf5-py/src/select.rs
index 7d11f3c..93a1ad1 100644
--- a/crates/clawhdf5-py/src/select.rs
+++ b/crates/clawhdf5-py/src/select.rs
@@ -7,11 +7,12 @@
 //! (negative from the end) drop their axis, slices must have a positive
 //! step, one `Ellipsis` fills the unmentioned axes, a single increasing list
 //! of integers may index one axis, and strings name compound fields.
-//! Everything else (`None`/`np.newaxis`, boolean masks, several index lists)
-//! is refused with the error h5py gives.
+//! Everything else (`None`/`np.newaxis`, several index lists) is refused
+//! with the error h5py gives; boolean masks, which h5py supports, raise
+//! `NotImplementedError`.
 
 use clawhdf5_format::selection::Selection;
-use pyo3::exceptions::{PyIndexError, PyTypeError, PyValueError};
+use pyo3::exceptions::{PyIndexError, PyNotImplementedError, PyTypeError, PyValueError};
 use pyo3::prelude::*;
 use pyo3::types::{PyEllipsis, PySlice, PyString, PyTuple};
 
@@ -379,8 +380,10 @@ fn parse_axis(py: Python<'_>, a: &Bound<'_, PyAny>, n: u64) -> PyResult<Axis> {
         let arr = np.call_method1("asarray", (a,))?;
         let kind: String = arr.getattr("dtype")?.getattr("kind")?.extract()?;
         if kind == "b" {
-            return Err(PyTypeError::new_err(
-                "Boolean mask indexing is not supported by clawhdf5",
+            // h5py supports masks; clawhdf5 does not (yet), for reads or
+            // writes: an unsupported operation, not a wrong key.
+            return Err(PyNotImplementedError::new_err(
+                "boolean mask indexing is not supported by clawhdf5",
             ));
         }
         let ndim: usize = arr.getattr("ndim")?.extract()?;
diff --git a/crates/clawhdf5-py/tests/test_edit.py b/crates/clawhdf5-py/tests/test_edit.py
index ac28607..a49df60 100644
--- a/crates/clawhdf5-py/tests/test_edit.py
+++ b/crates/clawhdf5-py/tests/test_edit.py
@@ -643,6 +643,13 @@ def test_unsupported_edits_are_clear_errors(h5py, tmp_path):
             f["chunk_ext"].resize(3, axis=2)
         with pytest.raises(TypeError):
             f["i4"][0] = np.array(["a"] * 10)
+        # h5py writes through boolean masks; clawhdf5 does not.
+        with pytest.raises(NotImplementedError, match="mask"):
+            f["u1"][np.arange(16) % 2 == 0] = 5
+        with pytest.raises(NotImplementedError, match="mask"):
+            f["i4"][f["i4"][()] > 30] = 0
+        with pytest.raises(NotImplementedError, match="mask"):
+            f["i4"][np.ones(6, dtype=bool), 2] = 0
     assert snapshot(h5py, path) == before
     h5dump_reads(path)
 
diff --git a/crates/clawhdf5-py/tests/test_read_vs_h5py.py b/crates/clawhdf5-py/tests/test_read_vs_h5py.py
index 4fab9d2..d45dd61 100644
--- a/crates/clawhdf5-py/tests/test_read_vs_h5py.py
+++ b/crates/clawhdf5-py/tests/test_read_vs_h5py.py
@@ -440,7 +440,7 @@ def test_unsupported_types_are_errors_not_data(pair):
 
 def test_boolean_masks_are_refused(pair):
     ours, _, _ = pair
-    with pytest.raises(TypeError):
+    with pytest.raises(NotImplementedError, match="mask"):
         ours["num/le_i4_1d"][np.ones(37, dtype=bool)]
 
 
diff --git a/docs/known-issues.md b/docs/known-issues.md
index 871729d..cc2015e 100644
--- a/docs/known-issues.md
+++ b/docs/known-issues.md
@@ -164,8 +164,9 @@ before anything is written. On top of them:
   elements, variable-length data, strings padded with spaces or
   NUL-terminated (libhdf5 converts those differently from numpy; NUL-padded
   ones, h5py's, are writable), compounds containing such strings, null
-  dataspaces, and index-list writes of more than 2²² elements (write them
-  in slices).
+  dataspaces, index-list writes of more than 2²² elements (write them
+  in slices), and boolean-mask keys (`ds[mask] = v`, and mask reads;
+  h5py supports both).
 - **`str` attributes are fixed-length UTF-8**, where h5py writes
   variable-length strings: h5py reads them back as `bytes`
   (`numpy.bytes_`), not `str`.