Knockout绑定字符串组合整数，为什么未定义？

Question

我的viewModel this ..

function ViewModel() {
var self = this;
self.model = {};
self.Message = ko.observable("");
self.model.CurrentDisplayAmr = ko.observable();
self.model.SelectedAmr = ko.observable();
self.model.CurrentAmrWirings = ko.observableArray([]);
self.model.CurrentSelectedWire = ko.observable();
self.model.AmrCommands = ko.observableArray([]);

//selected Amr is update here
self.selectAmr = function (item) {
    self.model.SelectedAmr(item);        
    self.GetAmrWirings();
};

//Amrs search is here
self.GetAmrs = function (searchTerm) {
    var self = this; // Retain scope of view model
    $.ajax({
        url: '/AmrSearch/SearchAmrs',
        cache: false,
        contentType: 'application/json',
        data: ko.toJSON({ AmrNo: self.SearchAmrNo }),
        type: "POST",
        success: function (result) {                
            $('#processing-modal').modal('hide');

            if (result.success) {
                self.model.CurrentDisplayAmr(result.data);
                self.Message(result.message);
            }
            else
                self.Message("Modem aramada sorun oluştu: " + result.message);

        },
        error: function (errorDetail) {
            $('#processing-modal').modal('hide');
            self.Message("Modem arama isteğinde hata: " + errorDetail);
        }
    });
}

Amr Selection html is here :
    <form class="form-horizontal">
        <fieldset>
            <div class="col-md-4">
                <div class="input-group">
                    <input disabled type="text" class="form-control" placeholder="0000000" data-bind="value: model.CurrentDisplayAmr.AmrRemoteId">
                    <span class="input-group-btn">
                        <button class="btn btn-default" type="button" data-bind="click: $root.selectAmr" data-toggle="modal" data-target="#processing-modal">
                            <span class="glyphicon glyphicon-ok"></span> Seç
                        </button>
                    </span>
                </div>
            </div>
        </fieldset>

    </form>

如果我绑定文本 self.model.SelectedAmr.AmrRemoteId 没问题。

这段代码还可以：

  <span class="glyphicon glyphicon-hdd" style="font-size: 38px;"></span> <a data-bind="text:model.SelectedAmr.AmrRemoteId"></a>

结果是： 的 00000012

但如果我改变了这个：

  <span class="glyphicon glyphicon-hdd" style="font-size: 38px;"></span> <a data-bind="text:'Selected Modem ' + model.SelectedAmr.AmrRemoteId"></a>

结果是

所选调制解调器未定义

为什么不能组合字符串和数值？