diff --git a/metomi/isodatetime/data.py b/metomi/isodatetime/data.py
index bf14e28..bef2a3e 100644
--- a/metomi/isodatetime/data.py
+++ b/metomi/isodatetime/data.py
@@ -552,19 +552,17 @@ def minutes(self): return self._minutes
     @property
     def seconds(self): return self._seconds
 
-    def _copy(self):
+    def _copy(self) -> 'Duration':
         """Return an (unlinked) copy of this instance."""
         new = self.__class__(_is_empty_instance=True)
         for attr in self.__slots__:
             setattr(new, attr, getattr(self, attr))
         return new
 
-    def is_exact(self):
+    def is_exact(self) -> bool:
         """Return True if the instance is defined in non-nominal/exact units
         (weeks, days, hours, minutes or seconds) only."""
-        if self._years or self._months:
-            return False
-        return True
+        return not (self._years or self._months)
 
     def get_days_and_seconds(self):
         """Return a roughly-converted duration in days and seconds.
diff --git a/metomi/isodatetime/tests/test_01.py b/metomi/isodatetime/tests/test_01.py
index 7adab88..e562b6b 100644
--- a/metomi/isodatetime/tests/test_01.py
+++ b/metomi/isodatetime/tests/test_01.py
@@ -612,6 +612,13 @@ def test_duration_subtract(self):
             self.assertEqual(test_subtract, end_point,
                              "%s - %s" % (start_point, test_duration))
 
+    def test_duration_is_exact(self):
+        """Test Duration.is_exact()."""
+        duration = data.Duration(weeks=1, days=1)
+        assert duration.is_exact()
+        for duration in (data.Duration(months=1), data.Duration(years=1)):
+            assert not duration.is_exact()
+
     def test_timepoint_comparison(self):
         """Test the TimePoint rich comparison methods and hashing."""
         run_comparison_tests(data.TimePoint, get_timepoint_comparison_tests())