diff --git a/tests/unit_tests/cad/__init__.py b/tests/unit_tests/cad/__init__.py
new file mode 100644
index 0000000000..59a520c12a
--- /dev/null
+++ b/tests/unit_tests/cad/__init__.py
@@ -0,0 +1,9 @@
+import numpy as np
+import pytest
+
+
+def assert_unbounded(obj):
+    """Assert that a region/cell is unbounded."""
+    ll, ur = obj.bounding_box
+    assert ll == pytest.approx((-np.inf, -np.inf, -np.inf))
+    assert ur == pytest.approx((np.inf, np.inf, np.inf))
diff --git a/tests/unit_tests/cad/test_cad.py b/tests/unit_tests/cad/test_cad.py
index b6a8db9bd5..5df861aa1d 100644
--- a/tests/unit_tests/cad/test_cad.py
+++ b/tests/unit_tests/cad/test_cad.py
@@ -1,8 +1,6 @@
 
-import pytest
-
 from subprocess import call
 
 def test_cad():
-    assert not call(["openmc"])
+    assert not call(['openmc'])