diff --git a/openmc/plots.py b/openmc/plots.py
index 7532d9d5cb..2bb3ef65f4 100644
--- a/openmc/plots.py
+++ b/openmc/plots.py
@@ -250,9 +250,10 @@ def voxel_to_vtk(voxel_file: PathLike, output: PathLike = 'plot.vti'):
         writer.SetInputData(grid)
     else:
         writer.SetInput(grid)
+    output = str(output)
     if not output.endswith(".vti"):
         output += ".vti"
-    writer.SetFileName(str(output))
+    writer.SetFileName(output)
     writer.Write()
 
     return output
diff --git a/tests/unit_tests/test_plots.py b/tests/unit_tests/test_plots.py
index a1e15016a2..4efc12c932 100644
--- a/tests/unit_tests/test_plots.py
+++ b/tests/unit_tests/test_plots.py
@@ -90,7 +90,7 @@ def test_voxel_plot(run_in_tmpdir):
     assert Path('test_voxel_plot.vti').is_file()
 
     vox_plot.filename = 'h5_voxel_plot'
-    vox_plot.to_vtk('another_test_voxel_plot.vti')
+    vox_plot.to_vtk(Path('another_test_voxel_plot.vti'))
 
     assert Path('h5_voxel_plot.h5').is_file()
     assert Path('another_test_voxel_plot.vti').is_file()