diff --git a/src/dask_awkward/lib/unproject_layout.py b/src/dask_awkward/lib/unproject_layout.py
index 0e1db0ad..493e3150 100644
--- a/src/dask_awkward/lib/unproject_layout.py
+++ b/src/dask_awkward/lib/unproject_layout.py
@@ -316,8 +316,15 @@ def _unproject_layout(form, layout, length, backend):
     # UnmaskedArray, non-UnmaskedArray form
     elif isinstance(layout, UnmaskedArray) and form.is_option:
         if isinstance(form, BitMaskedForm):
+            byte_length = (
+                unknown_length if length is unknown_length else math.ceil(length / 8.0)
+            )
             return BitMaskedArray(
-                ak.index.IndexU8.zeros(length, backend.index_nplike),
+                ak.index.Index(
+                    backend.index_nplike.full(byte_length, 255, dtype=np.uint8)
+                    if form.valid_when
+                    else backend.index_nplike.zeros(byte_length, dtype=np.uint8)
+                ),
                 _unproject_layout(
                     form.content, layout.content, layout.content.length, backend
                 ),
@@ -328,7 +335,11 @@ def _unproject_layout(form, layout, length, backend):
             )
         elif isinstance(form, ByteMaskedForm):
             return ByteMaskedArray(
-                ak.index.Index8.zeros(length, backend.index_nplike),
+                ak.index.Index(
+                    backend.index_nplike.full(length, 1, dtype=np.int8)
+                    if form.valid_when
+                    else backend.index_nplike.zeros(length, dtype=np.int8)
+                ),
                 _unproject_layout(
                     form.content, layout.content, layout.content.length, backend
                 ),
diff --git a/tests/test_unproject_layout.py b/tests/test_unproject_layout.py
index d5641725..552a5e1a 100644
--- a/tests/test_unproject_layout.py
+++ b/tests/test_unproject_layout.py
@@ -87,3 +87,24 @@ def test_UnionArray():
     projected = ak.from_iter([{"x": 1}, {"x": 2}, {"x": 3}], highlevel=False)
     unprojected = unproject_layout(form, projected)
     compare_values(projected, unprojected)
+
+
+def test_BitMaskedArray():
+    form = ak.forms.BitMaskedForm('u8', ak.forms.NumpyForm('int64'), valid_when=False, lsb_order=True)
+    projected = ak.contents.UnmaskedArray(ak.from_iter([1, 2, 3, 4], highlevel=False))
+    unprojected = unproject_layout(form, projected)
+    compare_values(projected, unprojected)
+
+    form = ak.forms.BitMaskedForm('u8', ak.forms.NumpyForm('int64'), valid_when=True, lsb_order=True)
+    unprojected = unproject_layout(form, projected)
+    compare_values(projected, unprojected)
+
+def test_ByteMaskedArray():
+    form = ak.forms.ByteMaskedForm('u8', ak.forms.NumpyForm('int64'), valid_when=False)
+    projected = ak.contents.UnmaskedArray(ak.from_iter([1, 2, 3, 4], highlevel=False))
+    unprojected = unproject_layout(form, projected)
+    compare_values(projected, unprojected)
+
+    form = ak.forms.ByteMaskedForm('u8', ak.forms.NumpyForm('int64'), valid_when=True)
+    unprojected = unproject_layout(form, projected)
+    compare_values(projected, unprojected)