diff --git a/openmc/geometry.py b/openmc/geometry.py
index 4cbc60c24..e939ecbe3 100644
--- a/openmc/geometry.py
+++ b/openmc/geometry.py
@@ -266,7 +266,10 @@ class Geometry(object):
             Dictionary mapping cell IDs to :class:`openmc.Cell` instances
 
         """
-        return self.root_universe.get_all_cells()
+        if self.root_universe is not None:
+            return self.root_universe.get_all_cells()
+        else:
+            return []
 
     def get_all_universes(self):
         """Return all universes in the geometry.
diff --git a/tests/regression_tests/dagmc/results_true.dat b/tests/regression_tests/dagmc/results_true.dat
new file mode 100644
index 000000000..fc13a9cda
--- /dev/null
+++ b/tests/regression_tests/dagmc/results_true.dat
@@ -0,0 +1,5 @@
+k-combined:
+1.115067E+00 5.423808E-02
+tally 1:
+8.543144E+00
+1.530584E+01
diff --git a/tests/regression_tests/dagmc/test.py b/tests/regression_tests/dagmc/test.py
index 01608d7ca..8c81b85d5 100644
--- a/tests/regression_tests/dagmc/test.py
+++ b/tests/regression_tests/dagmc/test.py
@@ -1,4 +1,4 @@
-from tests.testing_harness import CADTestHarness
+from tests.testing_harness import TestHarness
 import os
 import pytest
 
@@ -7,5 +7,5 @@ pytestmark = pytest.mark.skipif(
     reason="DAGMC CAD geometry is not enabled.")
 
 def test_dagmc():
-    harness = CADTestHarness('statepoint.5.h5')
+    harness = TestHarness('statepoint.5.h5')
     harness.main()    
diff --git a/tests/testing_harness.py b/tests/testing_harness.py
index 60c8e7c16..fb07575ce 100644
--- a/tests/testing_harness.py
+++ b/tests/testing_harness.py
@@ -312,15 +312,3 @@ class HashedPyAPITestHarness(PyAPITestHarness):
     def _get_results(self):
         """Digest info in the statepoint and return as a string."""
         return super()._get_results(True)
-
-
-class CADTestHarness(TestHarness):
-
-    def execute_test(self):
-        """Run OpenMC with the appropriate arguments and check the outputs."""
-        try:
-            self._run_openmc()
-            self._test_output_created()
-        finally:
-            self._cleanup()
-