diff --git a/openmc/mesh.py b/openmc/mesh.py
index 1ccb6e86e0..b883a4a096 100644
--- a/openmc/mesh.py
+++ b/openmc/mesh.py
@@ -690,7 +690,7 @@ class RegularMesh(StructuredMesh):
 
         # write the .vtk file
         writer = vtk.vtkStructuredGridWriter()
-        writer.SetFileName(filename)
+        writer.SetFileName(str(filename))
         writer.SetInputData(vtk_grid)
         writer.Write()
 
diff --git a/tests/unit_tests/test_mesh_to_vtk.py b/tests/unit_tests/test_mesh_to_vtk.py
index cfb0407ca0..66f99f65f4 100644
--- a/tests/unit_tests/test_mesh_to_vtk.py
+++ b/tests/unit_tests/test_mesh_to_vtk.py
@@ -33,7 +33,7 @@ def test_write_data_to_vtk(mesh, tmpdir):
     # BUILD
     filename = tmpdir / "out.vtk"
 
-    data = np.random.random(mesh.dimension[0]*mesh.dimension[1]*mesh.dimension[2])
+    data = np.random.random(mesh.num_mesh_cells)
 
     # RUN
     mesh.write_data_to_vtk(filename=filename, datasets={"label1": data, "label2": data})