spurious error message for compile that should fail

THE ERROR MEASSAGE IS:

WildcardCallRestrictions2.java:13: compiler message file broken: key=? super java.lang.String arguments={1}, {2}, {3}, {4}, {5}, {6}, {7}
                str = con.deref(); // <<fail>>


THE CODE IS:

 
public class WildcardCallRestrictions2
{
    public static void main(String[] args)
{
_Ref<? super String> con = null;
String str = "Your hair is on fire!";
str = con.deref(); // <<fail>>
    }
}

interface _Ref<Type>
{
    public void setDefined(boolean def);
    public boolean isDefined();
    public void set(Type val);
    public Type deref();
}