diff --git a/docs/iris/src/whatsnew/3.0.rst b/docs/iris/src/whatsnew/3.0.rst
index 59f7ec8735..72ccdf5066 100644
--- a/docs/iris/src/whatsnew/3.0.rst
+++ b/docs/iris/src/whatsnew/3.0.rst
@@ -168,7 +168,11 @@ This document explains the changes made to Iris for this release
   Previously, the first tick label would occasionally be duplicated. This also
   removes the use of Matplotlib's deprecated ``IndexFormatter``. (:pull:`3857`)
 
-* `@znicholls`_ fixed :meth:`~iris.quickplot._title` to only check ``units.is_time_reference`` if the ``units`` symbol is not used. (:pull:`3902`)
+* `@znicholls`_ fixed :meth:`~iris.quickplot._title` to only check 
+  ``units.is_time_reference`` if the ``units`` symbol is not used. (:pull:`3902`)
+
+* `@rcomer`_ fixed a bug whereby numpy array type attributes on a cube's 
+  coordinates could prevent printing it.  See :issue:`3921`.  (:pull:`3922`)
 
 .. _whatsnew 3.0 changes:
 
diff --git a/lib/iris/cube.py b/lib/iris/cube.py
index 3d0854355c..daffe11835 100644
--- a/lib/iris/cube.py
+++ b/lib/iris/cube.py
@@ -2182,23 +2182,20 @@ def _summary_coord_extra(self, coord, indent):
         extra = ""
         similar_coords = self.coords(coord.name())
         if len(similar_coords) > 1:
-            # Find all the attribute keys
-            keys = set()
-            for similar_coord in similar_coords:
-                keys.update(similar_coord.attributes.keys())
-            # Look for any attributes that vary
+            similar_coords.remove(coord)
+            # Look for any attributes that vary.
             vary = set()
-            attributes = {}
-            for key in keys:
+            for key, value in coord.attributes.items():
                 for similar_coord in similar_coords:
                     if key not in similar_coord.attributes:
                         vary.add(key)
                         break
-                    value = similar_coord.attributes[key]
-                    if attributes.setdefault(key, value) != value:
+                    if not np.array_equal(
+                        similar_coord.attributes[key], value
+                    ):
                         vary.add(key)
                         break
-            keys = sorted(vary & set(coord.attributes.keys()))
+            keys = sorted(vary)
             bits = [
                 "{}={!r}".format(key, coord.attributes[key]) for key in keys
             ]
diff --git a/lib/iris/tests/unit/cube/test_Cube.py b/lib/iris/tests/unit/cube/test_Cube.py
index 01dfe365b4..72bb761cb4 100644
--- a/lib/iris/tests/unit/cube/test_Cube.py
+++ b/lib/iris/tests/unit/cube/test_Cube.py
@@ -484,6 +484,16 @@ def test_ancillary_variable(self):
         )
         self.assertEqual(cube.summary(), expected_summary)
 
+    def test_similar_coords(self):
+        coord1 = AuxCoord(
+            42, long_name="foo", attributes=dict(bar=np.array([2, 5]))
+        )
+        coord2 = coord1.copy()
+        coord2.attributes = dict(bar="baz")
+        for coord in [coord1, coord2]:
+            self.cube.add_aux_coord(coord)
+        self.assertIn("baz", self.cube.summary())
+
 
 class Test_is_compatible(tests.IrisTest):
     def setUp(self):