diff --git a/cf_units/__init__.py b/cf_units/__init__.py
index de170d65..ad71097c 100644
--- a/cf_units/__init__.py
+++ b/cf_units/__init__.py
@@ -1128,7 +1128,7 @@ def is_unknown(self):
             False
 
         """
-        return self.category == _CATEGORY_UNKNOWN
+        return self.category == _CATEGORY_UNKNOWN or self.ut_unit is None
 
     def is_no_unit(self):
         """
diff --git a/cf_units/tests/test_unit.py b/cf_units/tests/test_unit.py
index 286b6665..ebf42fc5 100644
--- a/cf_units/tests/test_unit.py
+++ b/cf_units/tests/test_unit.py
@@ -1,4 +1,4 @@
-# (C) British Crown Copyright 2010 - 2018, Met Office
+# (C) British Crown Copyright 2010 - 2019, Met Office
 #
 # This file is part of cf-units.
 #
@@ -747,6 +747,12 @@ def test_known_unit(self):
         u = Unit('meters')
         self.assertFalse(u.is_unknown())
 
+    def test_no_ut_pointer(self):
+        # Test that a unit that was poorly constructed has a
+        # degree of tolerance by making it unknown.
+        # https://github.com/SciTools/cf-units/issues/133 refers.
+        self.assertTrue(Unit.__new__(Unit).is_unknown())
+
 
 class Test_is_no_unit(unittest.TestCase):
 
diff --git a/cf_units/tests/unit/unit/test_Unit.py b/cf_units/tests/unit/unit/test_Unit.py
index 03dfa9b4..ea66e73f 100644
--- a/cf_units/tests/unit/unit/test_Unit.py
+++ b/cf_units/tests/unit/unit/test_Unit.py
@@ -1,4 +1,4 @@
-# (C) British Crown Copyright 2015 - 2018, Met Office
+# (C) British Crown Copyright 2015 - 2019, Met Office
 #
 # This file is part of cf-units.
 #
@@ -249,5 +249,14 @@ def test_not_time_unit(self):
         self.assertFalse(result)
 
 
+class Test_format(unittest.TestCase):
+    def test_invalid_ut_unit(self):
+        # https://github.com/SciTools/cf-units/issues/133 flagged up that
+        # format should be a little more tolerant of a Unit that has not been
+        # constructed correctly when using pytest.
+        unit = Unit.__new__(Unit)
+        self.assertEqual(unit.format(), 'unknown')
+
+
 if __name__ == '__main__':
     unittest.main()