"No header record was found" on Azure Data Lake

CsvExtractor.cs

using CsvHelper;
using Microsoft.Analytics.Interfaces;
using System;
using System.Collections.Generic;
using System.IO;
using System.Text;

namespace SomeNamespace
{
    public class CustomExtractor : CsvExtractor
    {
        public override IEnumerable<IRow> Extract(IUnstructuredReader input, IUpdatableRow outputrow)
        {
            string someValue = "QRS";

            using (var reader = new StreamReader(input.BaseStream))
            using (var csvReader = new CsvReader(reader))
            {
                while (csvReader.Read())
                {
                    if (csvReader.IsRecordEmpty()) continue;

                    var tokens = csvReader.CurrentRecord;

                    if (tokens[0].Contains("XYZ")) continue;

                    if (tokens[0].Contains("AB") || tokens[0].Contains("BC"))
                    {
                        someValue = tokens[0];
                        continue;
                    }

                    var someColumn = tokens[0];
                    outputrow.Set(0, someColumn);

                    yield return outputrow.AsReadOnly();
                }
            }
        }
    }
}

SomeScript.usql

REFERENCE ASSEMBLY master.CsvHelper;
REFERENCE ASSEMBLY master.SomeAssembly;

DECLARE @in = @"C:\SomeInput.csv";
DECLARE @in = @"C:\SomeOutput.csv";

@DATA =
    EXTRACT someColumn string
    FROM @in
    USING new SomeNamespace.CustomExtractor();

OUTPUT @DATA
TO @out
USING Outputters.Csv()