AcquireTokenAsync在Web API中使用UserAssertion失败

开发了一个使用ADAL的Web API。Net v3.14进行身份验证。下面是获取access\u令牌的代码(使用ADAL提供的默认令牌缓存)

var provider = "https://login.microsoftonline.com/XXXXXXXX.onmicrosoft.com"
var service = "https://XXXXXXXX.onmicrosoft.com/XXXXXXService" //which is registered as service in Azure AD
var clientId  = "01d2b529-XXXX-XXXX-b794-XXXXXXXXXXXX" //client app registered on Azure AD
AuthenticationContext authContext = new AuthenticationContext(provider);
UserPasswordCredential uc = new UserPasswordCredential(user, password);
AuthenticationResult result = authContext.AcquireTokenAsync(service, clientId, uc).ConfigureAwait(false).GetAwaiter().GetResult();

它成功返回Access_令牌。现在,在该令牌过期1小时后,我已经实现了以下代码,以使用Refresh\u令牌更新它(假设Refresh\u令牌将作为实现的ADAL令牌缓存从缓存中获取):

  AuthenticationContext authContext = new AuthenticationContext(provider);
  UserAssertion userAssertion = new UserAssertion(oldtoken, "urn:ietf:params:oauth:grant-type:jwt-bearer", upn);
  AuthenticationResult result = authContext.AcquireTokenAsync(resource,clientId, userAssertion).ConfigureAwait(false).GetAwaiter().GetResult();
  var token = result.AccessToken

此代码给出错误:

“无效的JWT令牌。AADSTS50027:无效的JWT令牌。令牌格式无效”。

我检查了“oldtoken”变量,它是有效的JWT令牌。