diff --git a/openmc/material.py b/openmc/material.py
index 2c633a5ac5..6ac153c866 100644
--- a/openmc/material.py
+++ b/openmc/material.py
@@ -982,7 +982,7 @@ class Material(IDManagerMixin):
             for sab in self._sab:
                 subelement = ET.SubElement(element, "sab")
                 subelement.set("name", sab[0])
-                subelement.set("fraction", sab[1])
+                subelement.set("fraction", str(sab[1]))
 
         return element