Threading Concept WaitHandle[] executing Error

[WebMethod]
  public XmlDocument GIHotelInventory(string GIMonth, string GIYear, string username)
    {
        XmlDocument Ctxml = new XmlDocument();
        XmlDocument Mmtxml = new XmlDocument();
        XmlDocument xml = new XmlDocument();

        var dt = ProviderDetails(username);

        CtCaller ct = new CtCaller(*****Inventory);
        MmtCaller mmt = new MmtCaller(*****Inventory);
        IAsyncResult ctreslt = null;
        IAsyncResult mmtreslt = null;
        string ProviderUserName = string.Empty;
        string ProviderPassword = string.Empty;
        foreach (DataRow a in dt.Rows)
        {
            CollectionProviderDetails prd = new CollectionProviderDetails();
            prd.GiUserName = Convert.ToString(a["GiUserName"]);
            prd.ProviderName = Convert.ToString(a["ProviderName"]);
            ProviderUserName = Convert.ToString(a["PrUserName"]);
            ProviderPassword = Convert.ToString(a["PrPassword"]);

            if (prd.ProviderName == Portal.*****.ToString())
            {
                ctreslt = ct.BeginInvoke(GIMonth, GIYear, ProviderUserName, ProviderPassword, null, null);

            }

            else if (prd.ProviderName == Portal.*****.ToString())
            {
                mmtreslt = mmt.BeginInvoke(GIMonth, GIYear, null, null);
            }
        }
        WaitHandle[] handleArray = { ctreslt.AsyncWaitHandle, mmtreslt.AsyncWaitHandle };
        WaitHandle.WaitAll(handleArray);
        Ctxml = ct.EndInvoke(ctreslt);
        Mmtxml = mmt.EndInvoke(mmtreslt);
        String Allxml = "<inventorydetails>\n";
        string GIxml = "<inventorydetails>\n" + Ctxml.InnerXml + " " + Mmtxml.InnerXml + "</inventorydetails>";
        Allxml += "</inventorydetails>";
        xml.LoadXml(GIxml);

        return xml;
    }