为什么我的KendoGrid“更新”参数在控制器中总是为空？

我的Index.cshtml文件中包含以下代码：

var dataSource = new kendo.data.DataSource({
    type: "json",
    transport: {
        read: {
            url: '@Url.Action("ReadTeachers", "EducationPortal")',
            dataType: "json"
        },
        update: {
            url: '@Url.Action("UpdateTeachers", "EducationPortal")',
            type: "POST"
        },
        parameterMap: function (data, operation) {
            if (operation != "read"){ 
                var result = {};

                for (var i = 0; i < data.models.length; i++) {
                    var teacher = data.models[i];
                    for (var member in teacher) {
                        result["teacher[" + i + "]." + member] = teacher[member];
                    }
                }

                return result;
            } else {
                return JSON.stringify(data);
            }
        }
    },
    batch: true,
    schema: {
        model: {
            id: "TeacherId",
            fields: {
                TeacherId: { type: "number" },
                FullName: { type: "string" },
                IsHeadmaster: { type: "boolean" }
            }
        }
    }
});

$("#teachers").kendoGrid({
    dataSource: dataSource,
    toolbar: ["create", "save"],
    columns: [
        { field: "FullName", title: "Teacher" },
        { field: "IsHeadmaster", title: "Is a Headmaster?", width: "120px" },
        { command: ["destroy"], title: "&nbsp;", width: "85px" }],
    editable: true
});

这是带有“批处理”编辑的标准KendoGrid。编辑和网格本身都可以正常工作，但后端却不能。当“更新”请求通过时，它将转到此控制器方法：

public void UpdateTeachers(string models)
{
    // this method will have real code later
    Console.Write(models);
}

当我在此处放置断点时，Visual Studio显示models为null。像这样：

为什么呢null？