Amazon MWS - request signature calculated does not match the signature provided

Question

Getting the following error message from https://mws.amazonservices.com/:

<Type>Sender</Type>
<Code>SignatureDoesNotMatch</Code>
−
<Message>
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.
</Message>

Here is the VB.net code I am using to calculate the request. I have removed the SecretKey and AWSAccessKeyId for security reasons.

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("<a href=""" & sURL & "?" & sRequest & """>Click here</a>")

    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