等待要求类型＆＃39; TaskofX＆＃39;有一个合适的GetAwaiter方法

Question

我构建了一个Web服务（＆＃34; InvStatus＆＃34;），它包含以下单个函数：

Imports System.Web
Imports System.Web.Services
Imports System.Web.Services.Protocols
Imports System.Threading.Tasks


<WebService(Namespace:="http://example.com/InvStatus/", Name:="InvStatus",
        Description:="the description")>
<WebServiceBinding(ConformsTo:=WsiProfiles.BasicProfile1_1)>
<Global.Microsoft.VisualBasic.CompilerServices.DesignerGenerated()>
Public Class InvStatus
Inherits System.Web.Services.WebService

Public Structure CustomReturnStruct
    Dim firstItem As String
    Dim secondItem As integer
End Structure


<WebMethod()>
  Public Async Function LookupAsync(ByVal itemCode As String) 
       As Task(Of CustomReturnStruct)

    Dim result As New CustomReturnStruct

    ' ... look up the item, and stuff the results into 
    '     result.firstItem and result.seconditem 
    ' ...

    return result
  End Function
End Class

服务编译正常，但当我尝试从另一个程序中调用它作为Web引用时：

dim myWebSvc As New com.example.InvStatus

--> Dim result As CustomReturnStruct = Await myWebSvc.LookupAsync("test123") <--

...给出了一个编译错误，说“等待”＃39;要求TaskOfCustomReturnStruct类型具有合适的GetAwaiter方法

我做错了什么？ （以及GetAwaiter方法是什么？）