Amazon Web service - signature

I've been receiving an error from Amazon web service - InvalidParameterValue Either Action or Operation query parameter must be present.

I believe it is most likely due to the signature being incorrect as the XML document and Header matches that of a test I did in their scratchpad.

Does anything stand out as being incorrect?

Thanks,

Clare

using System;
using System.Collections.Generic;
using System.Security.Cryptography;
using System.Text;
using System.Web;

public class SignatureHelper
{
    private static string ConstructCanonicalQueryString(SortedDictionary<string, string> sortedParameters)
    {
        var builder = new StringBuilder();

        if (sortedParameters.Count == 0)
        {
            builder.Append(string.Empty);
            return builder.ToString();
        }

        foreach (var kvp in sortedParameters)
        {
            builder.Append(PercentEncodeRfc3986(kvp.Key));
            builder.Append("=");
            builder.Append(PercentEncodeRfc3986(kvp.Value));
            builder.Append("&");
        }

        var canonicalString = builder.ToString();
        return canonicalString.Substring(0, canonicalString.Length - 1);
    }

    /// <summary>
    /// Percent-encode (URL Encode) according to RFC 3986 as required by Amazon.
    /// This is necessary because .NET's HttpUtility.UrlEncode does not encode
    /// * according to the above standard. Also, .NET returns lower-case encoding
    /// * by default and Amazon requires upper-case encoding.
    /// </summary>
    /// <param name="value"></param>
    /// <returns></returns>
    private static string PercentEncodeRfc3986(string value)
    {
        value = HttpUtility.UrlEncode(string.IsNullOrEmpty(value) ? string.Empty : value, Encoding.UTF8);

        if (string.IsNullOrEmpty(value))
        {
            return string.Empty;
        }

        value = value.Replace("'", "%27")
                .Replace("(", "%28")
                .Replace(")", "%29")
                .Replace("*", "%2A")
                .Replace("!", "%21")
                .Replace("%7e", "~")
                .Replace("+", "%20")
                .Replace(":", "%3A");

        var sbuilder = new StringBuilder(value);

        for (var i = 0; i < sbuilder.Length; i++)
        {
            if (sbuilder[i] != '%')
            {
                continue;
            }

            if (!char.IsLetter(sbuilder[i + 1]) && !char.IsLetter(sbuilder[i + 2]))
            {
                continue;
            }

            sbuilder[i + 1] = char.ToUpper(sbuilder[i + 1]);
            sbuilder[i + 2] = char.ToUpper(sbuilder[i + 2]);
        }

        return sbuilder.ToString();
    }

    public string SignRequest(Dictionary<string, string> parametersUrl, Dictionary<string, string> 

parametersSignture)
    {
        var secret = Encoding.UTF8.GetBytes(parametersSignture["Secret"]);
        var signer = new HMACSHA256(secret);

        var pc = new ParamComparer();
        var sortedParameters = new SortedDictionary<string, string>(parametersUrl, pc);
        var orderedParameters = ConstructCanonicalQueryString(sortedParameters);

        // Derive the bytes needs to be signed.
        var builder = new StringBuilder();
        builder.Append(parametersSignture["RequestMethod"])
                .Append(" \n")
                .Append(parametersSignture["EndPoint"])
                .Append("\n")
                .Append("/\n")
                .Append(orderedParameters);

        var stringToSign = builder.ToString();
        var toSign = Encoding.UTF8.GetBytes(stringToSign);

        // Compute the signature and convert to Base64.
        var sigBytes = signer.ComputeHash(toSign);
        var signature = Convert.ToBase64String(sigBytes);

        return signature.Replace("=", "%3D").Replace("/", "%2F").Replace("+", "%2B");
    }

    public class ParamComparer : IComparer<string>
    {
        public int Compare(string p1, string p2)
        {
            return string.CompareOrdinal(p1, p2);
        }
    }
}
}