diff --git a/openmc/mesh.py b/openmc/mesh.py
index 962751f9b3..3cdfc87f3b 100644
--- a/openmc/mesh.py
+++ b/openmc/mesh.py
@@ -795,7 +795,7 @@ class RegularMesh(StructuredMesh):
     def is_flat(self):
         """Returns True if the mesh is flat
         """
-        if None in [self.lower_left, self.upper_right]:
+        if self.lower_left is None or self.upper_right is None:
             return False
 
         for val1, val2 in zip(self.lower_left, self.upper_right):
diff --git a/tests/unit_tests/test_mesh.py b/tests/unit_tests/test_mesh.py
index 129e8e21b5..8bc00a2768 100644
--- a/tests/unit_tests/test_mesh.py
+++ b/tests/unit_tests/test_mesh.py
@@ -1,5 +1,6 @@
 import openmc
 import pytest
+import numpy as np
 
 @pytest.mark.parametrize("val_left,val_right", [(0, 0), (-1., -1.), (2.0, 2)])
 def test_raises_error_when_flat(val_left, val_right):
@@ -32,3 +33,25 @@ def test_raises_error_when_flat(val_left, val_right):
     with pytest.raises(ValueError):
         mesh.upper_right = [25, 25, val_right]
         mesh.lower_left = [-25, -25, val_left]
+
+
+def test_corner_none_returns_false():
+    """Checks mesh is not considered flat when one
+    corner is None
+    """
+    mesh = openmc.RegularMesh()
+    mesh.lower_left = [-25, -25, -25]
+    assert not mesh.is_flat()
+
+    mesh = openmc.RegularMesh()
+    mesh.upper_right = [-25, -25, -25]
+    assert not mesh.is_flat()
+
+    # test with np array
+    mesh = openmc.RegularMesh()
+    mesh.lower_left = np.array([-25, -25, -25])
+    assert not mesh.is_flat()
+
+    mesh = openmc.RegularMesh()
+    mesh.upper_right = np.array([-25, -25, -25])
+    assert not mesh.is_flat()
\ No newline at end of file