Amazon MWS - вычисленная подпись запроса не соответствует обеспеченной подписи

Получение следующего сообщения об ошибке от https://mws.amazonservices.com/:

Sender
SignatureDoesNotMatch
−

The request signature we calculated does not match the signature you provided. Check your AWS Secret Access Key and signing method. Consult the service documentation for details.

Вот код VB.net, который я использую для вычисления запроса. Я удалил SecretKey и AWSAccessKeyId из соображений безопасности.

Protected Sub Page_Load(ByVal sender As Object, ByVal e As System.EventArgs) Handles Me.Load
        Dim sURL As String = "https://mws.amazonservices.com/"

        Dim sRequest As String = ""
        sRequest &= "Acknowledged=" & Server.UrlEncode("false")
        sRequest &= "&Action=" & Server.UrlEncode("GetReportList")
        sRequest &= "&AWSAccessKeyId=" & Server.UrlEncode("REMOVED-FOR-SECURITY")
        sRequest &= "&Marketplace=" & Server.UrlEncode("REMOVED-FOR-SECURITY")
        sRequest &= "&Merchant=" & Server.UrlEncode("REMOVED-FOR-SECURITY")
        sRequest &= "&SignatureMethod=" & Server.UrlEncode("HmacSHA256")
        sRequest &= "&SignatureVersion=" & Server.UrlEncode("2")
        sRequest &= "&Timestamp=" & Server.UrlEncode(DateTime.Now.ToString("yyyy-MM-ddTHH:mm:ssCST"))
        sRequest &= "&Version=" & Server.UrlEncode("2009-01-01")

        Dim StringToSign As String = "GET\n" & "mws.amazonservices.com\n" & "/\n" & sRequest
        sRequest &= "&Signature=" & Server.UrlEncode(HashString(StringToSign))

        Response.Write("Click here")

    End Sub

    Public Shared Function HashString(ByVal StringToHash As String) As String
        Dim myEncoder As New System.Text.UTF8Encoding
        Dim Key() As Byte = myEncoder.GetBytes("REMOVED-FOR-SECURITY")
        Dim XML() As Byte = myEncoder.GetBytes(StringToHash)
        Dim myHMACSHA256 As New System.Security.Cryptography.HMACSHA256(Key)
        Dim HashCode As Byte() = myHMACSHA256.ComputeHash(XML)
        Return Convert.ToBase64String(HashCode)
    End Function