angular ajax：JSON中位置0处的意外标记I

我想SENThtml如json在http post request。但是，当我尝试发出$http.post请求时，出现以下错误：

angular.js:12783 SyntaxError: Unexpected token I in JSON at position 0
    at Object.parse (native)
    at fromJson (http://localhost:9001/bower_components/angular/angular.js:1274:14)
    at defaultHttpResponseTransform (http://localhost:9001/bower_components/angular/angular.js:9703:16)
    at http://localhost:9001/bower_components/angular/angular.js:9794:12
    at forEach (http://localhost:9001/bower_components/angular/angular.js:341:20)
    at transformData (http://localhost:9001/bower_components/angular/angular.js:9793:3)
    at transformResponse (http://localhost:9001/bower_components/angular/angular.js:10582:21)
    at processQueue (http://localhost:9001/bower_components/angular/angular.js:15097:28)
    at http://localhost:9001/bower_components/angular/angular.js:15113:27
    at Scope.$eval (http://localhost:9001/bower_components/angular/angular.js:16359:28)

我的代码（角度）如下：

  $scope.generate_pdf = function() {
    var html =  angular.element('html').html(); // get all html

    var service = API.getService();
    // JSON.stringify( { html:  html } this also cause same error
    service.downloadPdf({}, { html: html },
      function(res) {
        console.log("res : ", res);
      }, function(err) {
        console.log("err : ", err);
      });


  };

我怎么解决这个问题？提前致谢