Kendo ui: Uncaught TypeError: Cannot read property 'toLowerCase' of undefined

I am using kendo ui treelist component, and fetching data from a remote source. The data source definition is :

$("#location-manage-grid").kendoTreeList({
    dataSource: new kendo.data.HierarchicalDataSource({
        transport: {
            read: {
                url: "api/getLocation",
                dataType: "json"
            }
        },
        schema: {
            model: {
                id: "id",
                location: "location",
                hierarchy: "hierarchy",
                hasChildren: "hasChildren",
                map: "map"
            }
        }
    }),
    height: 600,
    pageable: true,
    columns: [
        { field: "hierarchy", title: "Hierarchy", width: 250 },
        { field: "location", title: "location", width: 250 },
        { field: "map", title: "map", width: 250 },
        { title: "Edit", command: [ "edit", "destroy" ], width: 250,  attributes: { style: "text-align: center;" } }
    ]
});

The response of the datasource url is

[{"hasChildren":true,"hierarchy":"Kendo ui","location":"New York","id":1,"map":true}]

However, when I run the application, it has an error, which said:

Uncaught TypeError: Cannot read property 'toLowerCase' of undefined

I have checked the schema definition, and I really can't find where goes wrong. Does anyone know why?