diff --git a/lib/iris/tests/unit/data_manager/test_DataManager.py b/lib/iris/tests/unit/data_manager/test_DataManager.py
index 58004b120e..8e6df63158 100644
--- a/lib/iris/tests/unit/data_manager/test_DataManager.py
+++ b/lib/iris/tests/unit/data_manager/test_DataManager.py
@@ -184,6 +184,7 @@ def setUp(self):
 
     def test_array_none(self):
         self.dm._real_array = None
+        self.dm._shape = None
         emsg = "Unexpected data state, got no lazy or real data, and no shape."
         with self.assertRaisesRegex(ValueError, emsg):
             self.dm._assert_axioms()