fix: Properly slice validity mask on pl.Object series (#18631)

crates/polars-core/src/chunked_array/object/mod.rs

@@ -183,7 +183,11 @@ where
 
     unsafe fn slice_unchecked(&mut self, offset: usize, length: usize) {
         let len = std::cmp::min(self.len - offset, length);
-
+        self.null_bitmap = self
+            .null_bitmap
+            .take()
+            .map(|bitmap| bitmap.sliced_unchecked(offset, length))
+            .filter(|bitmap| bitmap.unset_bits() > 0);
         self.len = len;
         self.offset = offset;
     }

py-polars/tests/unit/datatypes/test_object.py

@@ -7,6 +7,7 @@
 
 import polars as pl
 from polars.exceptions import ComputeError
+from polars.testing import assert_series_equal
 
 
 def test_series_init_instantiated_object() -> None:
@@ -190,3 +191,11 @@ def test_raise_list_object() -> None:
     # We don't want to support this. Unsafe enough as it is already.
     with pytest.raises(ValueError):
         pl.Series([[object()]], dtype=pl.List(pl.Object()))
+
+
+def test_object_null_slice() -> None:
+    s = pl.Series("x", [1, None, 42], dtype=pl.Object)
+    assert_series_equal(s.is_null(), pl.Series("x", [False, True, False]))
+    assert_series_equal(s.slice(0, 2).is_null(), pl.Series("x", [False, True]))
+    assert_series_equal(s.slice(1, 1).is_null(), pl.Series("x", [True]))
+    assert_series_equal(s.slice(2, 1).is_null(), pl.Series("x", [False]))