Exception while retrieving documents saved in Sql as binary - Using C#

I'm using this code for retrieving documents saved in Sql as binary. I'm getting this exception:

Invalid attempt to GetBytes on column ''. The GetBytes function can only be used on columns of type Text, NText, or Image.

What do I need to change?

using (SqlConnection conn = new SqlConnection(connStr))
        {
            conn.Open();
            using (SqlCommand cmd = conn.CreateCommand())
            {

                cmd.CommandText = "SELECT fData.PathName(), GET_FILESTREAM_TRANSACTION_CONTEXT(), fName FROM MyFsTable where fId = @fId";
                DataGridViewRow row = this.DgDocuments.SelectedRows[0];
                int id = (int)row.Cells["fId"].Value;
                cmd.Parameters.Add("@fId", SqlDbType.Int).Value = id;
                using (SqlDataReader dr = cmd.ExecuteReader())
                {
                    while (dr.Read())
                    {
                        int size = 1024 * 1024;
                        byte[] buffer = new byte[size];
                        int readBytes = 0;
                        int index = 0;

                        using (FileStream fs = new FileStream(fileName, FileMode.Create, FileAccess.Write, FileShare.None))
                        {
                            while ((readBytes = (int)dr.GetBytes(0, index, buffer, 0, size)) > 0)
                            {
                                fs.Write(buffer, 0, readBytes);
                                index += readBytes;
                            }
                        }
                    }
                }
            }
        }