Saving custom member properties via API

Using Umbraco 6.0.5 I am trying to save some custom member properties via the API. The new member is created fine but the custom properties are not saved. I have checked that the properties exist. In fact if the property name is changed to one that does not exist an error is thrown:

 public override void Submit()
        {
            //get a membertype by its alias
            var mt = MemberType.GetByAlias(MemberTypeAlias); //needs to be an existing membertype
            //get the user(0)
            var user = new User(0);
            //create a new member with Member.MakeNew
            var member = Member.MakeNew(Name, mt, user);
            //assign email, password and loginname
            member.Email = Email;
            member.Password = Password;
            member.LoginName = Email;
            //asign custom properties
            member.getProperty("committee").Value = Committee;
            member.getProperty("organisation").Value = Organisation;
            
            //asssign a group, get the group by name, and assign its Id
            var group = MemberGroup.GetByName(MemberGroupName); //needs to be an existing MemberGroup
            member.AddGroup(group.Id);
            //generate the member xml with .XmlGenerate
            member.XmlGenerate(new System.Xml.XmlDocument());
            //add the member to the website cache to log the member in
            Member.AddMemberToCache(member);
        }