diff --git a/openmc/universe.py b/openmc/universe.py
index 94ea5624a..bdd6e6ec9 100644
--- a/openmc/universe.py
+++ b/openmc/universe.py
@@ -469,7 +469,7 @@ class Universe(UniverseBase):
         """
         nuclides = OrderedDict()
 
-        if self._atoms is not None:
+        if self._atoms:
             volume = self.volume
             for name, atoms in self._atoms.items():
                 nuclide = openmc.Nuclide(name)
diff --git a/tests/unit_tests/test_universe.py b/tests/unit_tests/test_universe.py
index ab3fd6e33..33c86b150 100644
--- a/tests/unit_tests/test_universe.py
+++ b/tests/unit_tests/test_universe.py
@@ -132,3 +132,14 @@ def test_create_xml(cell_with_lattice):
     assert all(c.get('universe') == str(u.id) for c in cell_elems)
     assert not (set(c.get('id') for c in cell_elems) ^
                 set(str(c.id) for c in cells))
+
+
+def test_get_nuclide_densities():
+    surf = openmc.Sphere()
+    material = openmc.Material()
+    material.add_elements_from_formula("H2O")
+    material.set_density("g/cm3", 1)
+    cell = openmc.Cell(region=-surf, fill=material)
+    universe = openmc.Universe(cells=[cell])
+    with pytest.raises(RuntimeError):
+        universe.get_nuclide_densities()