WebApi Route fails if streamcontentis not empty

Question

I'm trying to send an object using Web-APi 2 and protobuf-net. I keep getting a 404 error so I assume something goes wrong with the routing?

When I comment out the serialize line

ProtoBuf.Serializer.Serialize(memstream, package);

(so the memory stream stays empty) the routing works and RecievePackage is called. And the package parameter is empty. (the package object itsself is not NULL but all it's properties are.)

Whenever the memory stream is not empty I get a 404 error. What am I doing wrong here?

Receive code:

[Route("Receive")]
[HttpPost]
public async Task<IHttpActionResult> RecievePackage(PackageModel package)
{
    id = await SavePackage(package);

    return Created("", id);
}

Send code:

private async Task SyncUpAsync(string apiUrl, PackageModel package)
{
    using (HttpClient client = new HttpClient())
    using (var memstream = new MemoryStream())
    {
        var uri = new Uri(new Uri(ApiUrl), apiUrl);

        ProtoBuf.Serializer.Serialize(memstream, package);
        memstream.Position = 0;

        var content = new StreamContent(memstream);

        content.Headers.Add("SyncApiToken", ApiKey);
        content.Headers.Add("Content-Type", "application/x-protobuf");

        var response = await client.PostAsync(uri, content);
    }
}