WebTestRequest Outcome set to Fail

WebTestRequest Outcome set to Fail

WebTestRequest request4 = new
WebTestRequest("http://xxx.xxx.xxx.xxx:8080/ABC/test.jsp");
request4.ThinkTime = 5;
yield return request4;
String newSessionID = request4.Cookies["JSESSIONID"].ToString();
if (newSessionID != sessionID1){
request4.Outcome = Outcome.Fail;
}
request4 = null;
The WebTest has many requests and here's one of it. I had this problem
"The Outcome property of the WebTestRequest class may only be set in a
PostRequest event handler." WebTestException was unhandled by user code.
Could anyone advise what could be done?