diff --git a/tests/test_accessor.py b/tests/test_accessor.py
new file mode 100644
index 0000000..c938caf
--- /dev/null
+++ b/tests/test_accessor.py
@@ -0,0 +1,41 @@
+import numpy as np
+import pytest
+import xarray as xr
+
+import xarray_subset_grid.accessor  # noqa: F401 -- register accessor
+
+
+def test_accessor_warns_when_no_grid_recognized():
+    ds = xr.Dataset()
+    with pytest.warns(UserWarning, match="no grid type"):
+        accessor = ds.xsg
+    assert accessor.grid is None
+
+
+def test_subset_polygon_and_bbox_return_none_without_grid():
+    ds = xr.Dataset()
+    poly = np.array(
+        [
+            [-72.0, 41.0],
+            [-70.0, 41.0],
+            [-71.0, 39.0],
+            [-72.0, 41.0],
+        ]
+    )
+    with pytest.warns(UserWarning, match="no grid type"):
+        assert ds.xsg.subset_polygon(poly) is None
+        assert ds.xsg.subset_bbox((-72, 39, -70, 41)) is None
+
+
+def test_subset_vars_passthrough_without_grid():
+    ds = xr.Dataset({"a": (("x",), [1, 2, 3])})
+    with pytest.warns(UserWarning, match="no grid type"):
+        out = ds.xsg.subset_vars(["a"])
+    # Without a recognized grid, subset_vars returns the dataset unchanged.
+    assert "a" in out.data_vars
+
+
+def test_has_vertical_levels_false_without_grid():
+    ds = xr.Dataset()
+    with pytest.warns(UserWarning, match="no grid type"):
+        assert ds.xsg.has_vertical_levels is False
diff --git a/tests/test_grids/test_sgrid.py b/tests/test_grids/test_sgrid.py
index dce7379..5095699 100644
--- a/tests/test_grids/test_sgrid.py
+++ b/tests/test_grids/test_sgrid.py
@@ -8,6 +8,7 @@
 from xarray_subset_grid.grids.sgrid import _get_location_info_from_topology
 
 # open dataset as zarr object using fsspec reference file system and xarray
+zarr__version__ = 0
 try:
     import fsspec
     import zarr
diff --git a/tests/test_utils.py b/tests/test_utils.py
index 6b64a2c..6eb4a8c 100644
--- a/tests/test_utils.py
+++ b/tests/test_utils.py
@@ -1,9 +1,17 @@
 import os
+from datetime import datetime
 
+import cftime
 import numpy as np
 import pytest
+import xarray as xr
 
+from tests.conftest import EXAMPLE_DATA
+from xarray_subset_grid import utils as xsg_utils
 from xarray_subset_grid.utils import (
+    asdatetime,
+    compute_2d_subset_mask,
+    format_bytes,
     normalize_bbox_x_coords,
     normalize_polygon_x_coords,
     ray_tracing_numpy,
@@ -125,3 +133,72 @@ def test_ray_tracing_numpy():
     result = ray_tracing_numpy(points[:, 0], points[:, 1], poly)
 
     assert np.array_equal(result, [False, True, False])
+
+
+@pytest.mark.parametrize(
+    "num, unit",
+    [
+        (512, "bytes"),
+        (2048, "KB"),
+        (3 * 1024**2, "MB"),
+    ],
+)
+def test_format_bytes(num, unit):
+    assert unit in format_bytes(num)
+
+
+def test_asdatetime_none():
+    assert asdatetime(None) is None
+
+
+def test_asdatetime_datetime_passthrough():
+    dt = datetime(2020, 6, 15, 12, 30, 0)
+    assert asdatetime(dt) is dt
+
+
+def test_asdatetime_cftime_passthrough():
+    dt = cftime.datetime(2020, 6, 15, 12)
+    assert asdatetime(dt) is dt
+
+
+def test_asdatetime_parse_string():
+    dt = asdatetime("2020-06-15T12:30:00")
+    assert dt.year == 2020 and dt.month == 6 and dt.day == 15
+
+
+def test_compute_2d_subset_mask_all_inside():
+    ny, nx = 5, 5
+    lat = np.linspace(40.0, 44.0, ny)
+    lon = np.linspace(-74.0, -70.0, nx)
+    lat2d, lon2d = np.meshgrid(lat, lon, indexing="ij")
+    lat_da = xr.DataArray(lat2d, dims=("y", "x"))
+    lon_da = xr.DataArray(lon2d, dims=("y", "x"))
+    poly = np.array([(-75.0, 39.0), (-69.0, 39.0), (-69.0, 45.0), (-75.0, 45.0)])
+    mask = compute_2d_subset_mask(lat_da, lon_da, poly)
+    assert mask.dims == ("y", "x")
+    assert bool(mask.all())
+
+
+def test_compute_2d_subset_mask_partial():
+    ny, nx = 7, 7
+    lat = np.linspace(40.0, 46.0, ny)
+    lon = np.linspace(-74.0, -68.0, nx)
+    lat2d, lon2d = np.meshgrid(lat, lon, indexing="ij")
+    lat_da = xr.DataArray(lat2d, dims=("y", "x"))
+    lon_da = xr.DataArray(lon2d, dims=("y", "x"))
+    # Small polygon over the south-west corner only
+    poly = np.array([(-74.5, 40.0), (-73.0, 40.0), (-73.0, 41.0), (-74.5, 41.0)])
+    mask = compute_2d_subset_mask(lat_da, lon_da, poly)
+    assert mask.dims == ("y", "x")
+    assert bool(mask.any())
+    assert not bool(mask.all())
+
+
+def test_assign_ugrid_topology_utils_deprecation_wrapper():
+    nc = EXAMPLE_DATA / "SFBOFS_subset1.nc"
+    if not nc.is_file():
+        pytest.skip("example NetCDF not present")
+    ds = xr.open_dataset(nc)
+    with pytest.warns(DeprecationWarning, match="assign_ugrid_topology"):
+        ds2 = xsg_utils.assign_ugrid_topology(ds, face_node_connectivity="nv")
+    assert "mesh" in ds2.variables
diff --git a/xarray_subset_grid/utils.py b/xarray_subset_grid/utils.py
index e004525..da35800 100644
--- a/xarray_subset_grid/utils.py
+++ b/xarray_subset_grid/utils.py
@@ -19,12 +19,14 @@ def normalize_polygon_x_coords(x, poly):
     If the x coords are between 0 and 180 (i.e. both will work), the polygon
     is not changed.
 
-    NOTE: polygon is normalized in place!
+    NOTE: ``poly`` is normalized in place when it is already an ndarray;
+    a copy is made when ``poly`` is a sequence.
 
     Args:
         x (np.array): x-coordinates of the vertices
         poly (np.array): polygon vertices
     """
+    poly = np.asarray(poly)
     x_min, x_max = x.min(), x.max()
 
     poly_x = poly[:, 0]
@@ -97,10 +99,11 @@ def ray_tracing_numpy(x, y, poly):
 # this placeholder for backwards compatibility for a brief period
 def assign_ugrid_topology(*args, **kwargs):
     warnings.warn(
-        DeprecationWarning,
-        "The function `assign_grid_topology` has been moved to the "
+        "The function `assign_ugrid_topology` has been moved to the "
         "`grids.ugrid` module. It will not be able to be called from "
         "the utils `module` in the future.",
+        DeprecationWarning,
+        stacklevel=2,
     )
     from .grids.ugrid import assign_ugrid_topology