与Meteor.callLoginMethod匹配错误

我正在尝试用以下方式发送登录信息：

'```js submit form': function(event) { event.preventDefault(); event.stopPropagation();

var loginRequest = {
  username: event.target.loginUsername.value.toLowerCase(),
  password: event.target.loginPassword.value,
};

var callback = function(response) {
  Session.set('showLoading', false);
};

Session.set('showLoading', true);

Accounts.callLoginMethod({
  methodArguments: [loginRequest],
  userCallback: callback,
});

},

但是我收到一个错误，我不知道是什么导致了这个错误：

```js
Exception while invoking method 'login' Error: Match error: Unknown key in field username
     ...
 Sanitized and reported to the client as: Match failed [400]