diff --git a/tests/test_interval.py b/tests/test_interval.py
index 8c00796..df0fd4c 100644
--- a/tests/test_interval.py
+++ b/tests/test_interval.py
@@ -326,8 +326,13 @@ class TestInterval:
             list(diff2(a,b,True))
         list(diff2(a,b,False))
 
-        # Empty second set
-        eq_(nilmdb.utils.interval.set_difference(a, IntervalSet()), a)
+        # Fill out test coverage with a union operator (not implemented
+        # in interval.py, because nilmdb doesn't need it)
+        def union(a, b):
+            return nilmdb.utils.interval._interval_math_helper(
+                a, b, (lambda a, b: a or b), subset=False)
+        list(union(makeset("[---) "),
+                   makeset(" [---)")))
 
         # Empty second set
         eq_(nilmdb.utils.interval.set_difference(a, IntervalSet()), a)