diff --git a/src/wuttjamaican/diffs.py b/src/wuttjamaican/diffs.py
index 38e4214..47db028 100644
--- a/src/wuttjamaican/diffs.py
+++ b/src/wuttjamaican/diffs.py
@@ -83,7 +83,11 @@ class Diff:  # pylint: disable=too-many-instance-attributes
             self.cell_padding = cell_padding
 
     def make_fields(self):  # pylint: disable=missing-function-docstring
-        return sorted(set(self.old_data) | set(self.new_data), key=lambda x: x.lower())
+        if self.old_data and self.new_data:
+            fields = set(self.old_data) & set(self.new_data)
+        else:
+            fields = set(self.old_data or self.new_data)
+        return sorted(fields, key=lambda f: f.lower())
 
     def render_html(self, template=None, **kwargs):
         """
diff --git a/tests/test_diffs.py b/tests/test_diffs.py
index 32dedeb..dfb4452 100644
--- a/tests/test_diffs.py
+++ b/tests/test_diffs.py
@@ -19,13 +19,20 @@ class TestDiff(ConfigTestCase):
         self.assertEqual(diff.cell_padding, "0.5rem")
 
     def test_make_fields(self):
-        old_data = {"foo": "bar"}
-        new_data = {"foo": "bar", "baz": "zer"}
+
+        # should return "sorted intersection" of fields
+        old_data = {"foo": "bar", "baz": "abc"}
+        new_data = {"foo": "bar", "baz": "xyz", "flurg": "foo"}
         # nb. this calls make_fields()
         diff = self.make_diff(old_data, new_data)
-        # TODO: should the fields be cumulative? or just use new_data?
         self.assertEqual(diff.fields, ["baz", "foo"])
 
+        # all fields are used if only one data set
+        diff = self.make_diff(old_data, {})
+        self.assertEqual(diff.fields, ["baz", "foo"])
+        diff = self.make_diff({}, new_data)
+        self.assertEqual(diff.fields, ["baz", "flurg", "foo"])
+
     def test_values(self):
         old_data = {"foo": "bar"}
         new_data = {"foo": "baz"}