diff --git a/pyems/simulation.py b/pyems/simulation.py
index 80760ed..59fb1cf 100644
--- a/pyems/simulation.py
+++ b/pyems/simulation.py
@@ -234,7 +234,7 @@ def _mesh_errors(self) -> None:
     def save_csx(self, path: str = None):
         """"""
         if path is None:
-            path = self.sim_dir + "/" + self._file_name() + ".xml"
+            path = os.path.join(self.sim_dir, self._file_name() + ".xml")
         self.csx.Write2XML(path)
         self._csx_path = path