diff -up yum-3.4.3/yum/comps.py.old yum-3.4.3/yum/comps.py
--- yum-3.4.3/yum/comps.py.old	2013-05-02 11:02:28.000000000 -0400
+++ yum-3.4.3/yum/comps.py	2013-05-02 11:03:20.438942937 -0400
@@ -373,7 +373,10 @@ class Environment(CompsObj):
                 optionid = child.text
                 self._options[optionid] = 1
                 defopt = child.attrib.get('default')
-                default = parse_boolean(defopt)
+                if defopt:
+                    default = parse_boolean(defopt)
+                else:
+                    default = False
                 if default:
                     self._defaultoptions[optionid] = 1