Why cant I use two arguments in a WCF REST POST method?

I have the contract:

    [OperationContract]
    [WebInvoke(Method = "GET", ResponseFormat = WebMessageFormat.Xml, UriTemplate = "GetCategoriesGET/{userIdArg}", BodyStyle = WebMessageBodyStyle.Bare)]
    List<Video> GetVideosGET(string userIdArg);

    [WebInvoke(Method = "POST", UriTemplate = "evals")]
    [OperationContract]
    void SubmitVideoPOST(Video videoArg, string userId);

And I have the implementing methods:

public List<Video> GetVideosGET(string userIdArg)
{

  List<Video> catsToReturn = new List<Video>();

  if (Int32.Parse(userIdArg) == 1)
  {
      catsToReturn = catsForUser1;
  }
  else if (Int32.Parse(userIdArg) == 2)
  {
      catsToReturn = catsForUser2;
  }

  return catsToReturn;

  }


  public void SubmitVideoPOST(Video videoArg, string userId)
  {

  }

When I browse to:

http://localhost:52587/Api/Content/VLSContentService.svc/GetCategoriesGET/1

Im getting this error:

Server Error in '/' Application. Operation 'SubmitVideoPOST' of contract 'IVLSContentService' specifies multiple request body parameters to be serialized without any wrapper elements. At most one body parameter can be serialized without wrapper elements. Either remove the extra body parameters or set the BodyStyle property on the WebGetAttribute/WebInvokeAttribute to Wrapped.

I only started getting this error on the Get request when I added the new method for POST (which I havent tried to access), what does this mean? Cant I use more than one argument?