invalid attempt to read when no data is present, c#

Question: How can I correct the folowing error at line 18?

Error: invalid attempt to read when no data is present

After I changed lines 10 and 17, respectively, to read:

command.CommandText = "SELECT * From RodCtiteria";

if (reader.GetString(4) != null)

I got the error shown on the attached image with more detailed info.

public string ReadCriteria(string dataPoint)
    {
        string criteria = "";
        using (SqlConnection connection = new SqlConnection("Data Source=USER-PC;Initial Catalog=ROD_July18;Integrated Security=True"))
        {
            SqlCommand command = new SqlCommand();

            //string YYYY = rc.YYYY;// Session["YYYY"].ToString();
            command.Connection = connection;
            command.CommandText = "SELECT Top 1 [" + dataPoint + "] From RodCtiteria";
            command.CommandType = CommandType.Text;

            connection.Open();
            SqlDataReader reader = command.ExecuteReader();
            if (reader.HasRows)
            {
                if (reader.GetString(0) != "")
                    criteria = reader.GetString(0).ToString();     //<-- error here****************************
            }
            reader.Close();
        }
        return criteria;
    }

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readerError.png