Use Aws4RequestSigner to sign PAAPI 5 Request - amazon-web-services

Question

I'm trying to use Aws4RequestSigner in a VS2015 form to sign a search request to Amazon PAAPI.
https://www.nuget.org/packages/Aws4RequestSigner/
I get this response from the API:
{"__type":"com.amazon.paapi5#IncompleteSignatureException","Errors":[{"Code":"IncompleteSignature","Message":"The request signature did not include all of the required components. If you are using an AWS SDK, requests are signed for you automatically; otherwise, go to https://webservices.amazon.com/paapi5/documentation/sending-request.html#signing."}]}
private async void Form1_Load(object sender, EventArgs e)
{
_accessKey = "x";
_secretKey = "x";
_service = "ProductAdvertisingAPIv1";
_region = "us-east-1";
_requestUri = new Uri("https://webservices.amazon.com/paapi5/searchitems");
var payload = new
{
Keywords = "Harry",
Marketplace = "www.amazon.com",
PartnerTag = "x0d-20",
PartnerType = "Associates",
Resources = new string[] { "Images.Primary.Small", "ItemInfo.Title", "Offers.Listings.Price" },
SearchIndex = "All"
};
string jsonString = JsonConvert.SerializeObject(payload);
var content = new StringContent(jsonString, Encoding.UTF8, "application/json");
var xAmzDate = GetTimeStamp();
content.Headers.Add("content-encoding", "amz-1.0");
content.Headers.Add("x-amz-date", xAmzDate);
content.Headers.Add("x-amz-target", "com.amazon.paapi5.v1.ProductAdvertisingAPIv1.SearchItems");
var request = new HttpRequestMessage
{
Method = HttpMethod.Post,
RequestUri = _requestUri,
Content = content
};
request.Headers.Host = "webservices.amazon.com";
var contentType = new MediaTypeHeaderValue("application/json");
contentType.CharSet = "utf-8";
request.Content.Headers.ContentType = contentType;
var signer = new AWS4RequestSigner(_accessKey, _secretKey);
request = await signer.Sign(request, _service, _region);
try
{
var client = new HttpClient();
var response = await client.SendAsync(request);
if (!response.IsSuccessStatusCode)
{
var error = await response.Content.ReadAsStringAsync();
}
// response.EnsureSuccessStatusCode();
txtDisplay.Text = await response.Content.ReadAsStringAsync();
}
catch (HttpRequestException ex)
{
string error = ex.Message;
txtDisplay.Text = error;
}
}
private string GetTimeStamp()
{
return DateTime.UtcNow.ToString("yyyyMMdd\\THHmmss\\Z");
}
It could be that the headers are being added incorrectly or Aws4RequestSigner is simply outdated.