diff --git a/openmc/weight_windows.py b/openmc/weight_windows.py
index 2e5b7d8e07..257a17e9a5 100644
--- a/openmc/weight_windows.py
+++ b/openmc/weight_windows.py
@@ -291,10 +291,10 @@ class WeightWindows(IDManagerMixin):
         subelement.text = ' '.join(str(e) for e in self.energy_bounds)
 
         subelement = ET.SubElement(element, 'lower_ww_bounds')
-        subelement.text = ' '.join(str(b) for b in self.lower_ww_bounds)
+        subelement.text = ' '.join(str(b) for b in self.lower_ww_bounds.ravel())
 
         subelement = ET.SubElement(element, 'upper_ww_bounds')
-        subelement.text = ' '.join(str(b) for b in self.upper_ww_bounds)
+        subelement.text = ' '.join(str(b) for b in self.upper_ww_bounds.ravel())
 
         subelement = ET.SubElement(element, 'survival_ratio')
         subelement.text = str(self.survival_ratio)