diff --git a/lib/ncdata/dataset_like.py b/lib/ncdata/dataset_like.py
index 3702f95..b92ffa9 100644
--- a/lib/ncdata/dataset_like.py
+++ b/lib/ncdata/dataset_like.py
@@ -81,6 +81,11 @@ def __setattr__(self, attr, value):
         else:
             self.setncattr(attr, value)
 
+    def set_auto_chartostring(self, value):
+        if bool(value):
+            msg = "Cannot enable 'auto_chartostring' for Nc4Dataselike or Nc4VariableLike."
+            raise ValueError()
+
 
 class Nc4DatasetLike(_Nc4DatalikeWithNcattrs):
     """
diff --git a/tests/data_testcase_schemas.py b/tests/data_testcase_schemas.py
index 7622da3..ce68460 100644
--- a/tests/data_testcase_schemas.py
+++ b/tests/data_testcase_schemas.py
@@ -525,6 +525,7 @@ def standard_testcase(request, session_testdir):
             # """
             "small_rotPole_precipitation",
             "small_FC_167",
+            "test_monotonic_coordinate",
         ],
         # Xarray can save ~anything
         "save": [r"test_monotonic_coordinate"],
diff --git a/tests/integration/test_iris_xarray_roundtrips.py b/tests/integration/test_iris_xarray_roundtrips.py
index d0eb2bf..51ea827 100644
--- a/tests/integration/test_iris_xarray_roundtrips.py
+++ b/tests/integration/test_iris_xarray_roundtrips.py
@@ -79,9 +79,19 @@ def test_roundtrip_ixi(standard_testcase, use_irislock, adjust_chunks):
             "testdata____ugrid__21_triangle_example",
             # Problem with units on time bounds
             "label_and_climate__small_FC_167",
-            # Broken UGRID files now won't load in Iris >= 3.10
-            "unstructured_grid__mesh_C12",
-            "_unstructured_grid__theta_nodal_xios",
+            # # Broken UGRID files now won't load in Iris >= 3.10
+            # "unstructured_grid__mesh_C12",
+            # "_unstructured_grid__theta_nodal_xios",
+            # **No** mesh files will now roundtrip, since xarray 2026.04.0.
+            # This is because  xarray now normalises the array type when loading
+            # from ncdata, removing masks and converting to nanarrays (thus making ints
+            # into floats).
+            # Previously we could create xarray Datasets with masked integer variables
+            # as data -- though that was alwaysobviously a bit tricksy.
+            # Iris can't accept the new form as it insists on integer types for
+            # mesh connectivities :-(
+            # TODO: do something to get this working again?
+            "unstructured",
         ]
     )
     if any(key in standard_testcase.name for key in exclude_case_keys):