Gridview and "NULL" DateTime

I have a Gridview with an ObjectDataSource bound to it. The class handling the SelectMethod returns some DateTime fields that are for all purposes NULL, but actually DateTime.MinValue since a DateTime cannot be NULL.

I don't want to display these values in the Gridview, since they don't make any sense. Is it bad form to type a method as Object and return different types like so:

        public object Fax
        {
            get
            {
                if (fax.Date == DateTime.MinValue)
                {
                    return null;
                }
                else
                {
                    return fax;
                }
            }
            set { fax = (DateTime)value; }
        }

If it is, how could I inform the Gridview to hide the DateTime.MinValue?