diff --git a/src/utils/openmc/universe.py b/src/utils/openmc/universe.py
index e3280b3e3d..419d3bd10b 100644
--- a/src/utils/openmc/universe.py
+++ b/src/utils/openmc/universe.py
@@ -345,6 +345,9 @@ class Cell(object):
             element.set("fill", str(self._fill._id))
             self._fill.create_xml_subelement(xml_element)
 
+        elif self._fill == "void":
+            element.set("material", "void")
+
         else:
             element.set("fill", str(self._fill))
             self._fill.create_xml_subelement(xml_element)