diff --git a/lib/iris/cube.py b/lib/iris/cube.py
index a72d03eabc..1098b3ac86 100644
--- a/lib/iris/cube.py
+++ b/lib/iris/cube.py
@@ -2553,18 +2553,14 @@ def vector_summary(
                 )
                 summary += "\n     ugrid information:\n"
                 summary += "\n".join(ugrid_mesh_summary)
-                if ugrid_mesh.topology_dimension:
-                    summary += "\n{pad:{width}}topology_dimension: {val}".format(
-                        pad=" ",
-                        width=indent,
-                        val=ugrid_mesh.topology_dimension,
-                    )
-                if ugrid_mesh.node_coordinates:
-                    summary += "\n{pad:{width}}node_coordinates: {val}".format(
-                        pad=" ",
-                        width=indent,
-                        val=" ".join(ugrid_mesh.node_coordinates),
-                    )
+                summary += "\n{pad:{width}}topology_dimension: {val}".format(
+                    pad=" ", width=indent, val=ugrid_mesh.topology_dimension,
+                )
+                summary += "\n{pad:{width}}node_coordinates: {val}".format(
+                    pad=" ",
+                    width=indent,
+                    val=" ".join(ugrid_mesh.node_coordinates),
+                )
 
             #
             # Generate textual summary of cube scalar coordinates.
diff --git a/lib/iris/fileformats/ugrid_cf_reader.py b/lib/iris/fileformats/ugrid_cf_reader.py
index 7609588cc2..81e19f8bb8 100644
--- a/lib/iris/fileformats/ugrid_cf_reader.py
+++ b/lib/iris/fileformats/ugrid_cf_reader.py
@@ -57,7 +57,6 @@ class CubeUgrid(
             "topology_dimension",
             "node_coordinates",
         ],
-        defaults=(None,) * 2,
     )
 ):
     """
@@ -74,12 +73,11 @@ class CubeUgrid(
         Which element of the mesh the cube is mapped to.
         Can be 'face', 'edge' or 'node'.  A 'volume' is not supported.
 
-    Optional:
     * topology_dimension (int):
-        The highest dimensionality of the geometric elements in the mesh. Defaults to None.
+        The highest dimensionality of the geometric elements in the mesh.
 
-    * node_coordinates (string):
-        A list of the names of the spatial coordinates, used to geolocate the nodes. Defaults to None.
+    * node_coordinates (set):
+        A set of the names of the spatial coordinates, used to geolocate the nodes.
 
     """
 
@@ -88,14 +86,12 @@ def __str__(self):
         result += "\n   cube dimension = {}".format(self.cube_dim)
         result += '\n   mesh_location = "{}"'.format(self.mesh_location)
         result += '\n   mesh "{}" :'.format(self.grid.mesh_name)
-        if self.topology_dimension:
-            result += '\n   topology_dimension "{}" :'.format(
-                self.topology_dimension
-            )
-        if self.node_coordinates:
-            result += '\n   node_coordinates "{}" :\n'.format(
-                " ".join(self.node_coordinates)
-            )
+        result += '\n   topology_dimension "{}" :'.format(
+            self.topology_dimension
+        )
+        result += '\n   node_coordinates "{}" :\n'.format(
+            " ".join(self.node_coordinates)
+        )
         try:
             mesh_str = str(self.grid.info)
         except TypeError:
diff --git a/lib/iris/tests/unit/experimental/representation/test_CubeRepresentation.py b/lib/iris/tests/unit/experimental/representation/test_CubeRepresentation.py
index 6215c039be..3b25677e5e 100644
--- a/lib/iris/tests/unit/experimental/representation/test_CubeRepresentation.py
+++ b/lib/iris/tests/unit/experimental/representation/test_CubeRepresentation.py
@@ -126,8 +126,11 @@ def setUp(self):
         self.representer._get_bits(self.representer._get_lines())
 
     def test_population(self):
-        for v in self.representer.str_headings.values():
-            self.assertIsNotNone(v)
+        for k, v in self.representer.str_headings.items():
+            if k == "ugrid information:":
+                self.assertIsNone(v)
+            else:
+                self.assertIsNotNone(v)
 
     def test_headings__dimcoords(self):
         contents = self.representer.str_headings["Dimension coordinates:"]