diff --git a/openmc/mesh.py b/openmc/mesh.py
index 1372c5196e..e024710b48 100644
--- a/openmc/mesh.py
+++ b/openmc/mesh.py
@@ -699,7 +699,7 @@ class RegularMesh(StructuredMesh):
 
         # write the .vtk file
         writer = vtk.vtkStructuredGridWriter()
-        writer.SetFileName(filename)
+        writer.SetFileName(str(filename))
         writer.SetInputData(vtk_grid)
         writer.Write()