How to resolve issue of length is null or undefined while setting autocomplete text box in ASP.NET with C#?

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SQL

C#4.0

jQuery

I want to resolve issue of length is null or undefined while setting Autocomplete Text Box in asp.net with c#. I am setting the autocomplete based on Database in a content page. I am getting the exception
"Microsoft JScript runtime error: 'd.length' is null or not an object" on entering data in the Textbox.
The textbox in which autocomplete required is designed as below:

<div class="ui-widget">
Enter SFTP FileName: 
<asp:TextBox type="text" id="txtSearch" runat="server" class="autosuggest" >
</div>

What I have tried:

$(function () {
        SearchText();
    });
    function SearchText() {
        $(".autosuggest").autocomplete({
            source: function (request, response) {
                $.ajax({
                    type: "POST",
                    contentType: "application/json; charset=utf-8",
                    url: "AutoTB.aspx/GetAutoCompleteData",
                    data: "{'filename':'" + document.getElementById('MainContent_txtSearch').value + "'}",
                    dataType: "json",
                    success: function (data) {
                          if (data.d.length > 0) {
                            response($.map(data.d, function (item) {
                                return {
                                    label: item.split('/')[0],
                                    val: item.split('/')[1]
                                }
                            }));
                        }
                        else {
                            response([{ label: 'No Records Found', val: -1 }]);
                        }
                    },
                    error: function (result) {
                        alert("Error");
                    }
                });
            },
            select: function (event, ui) {
                if (ui.item.val == -1) {
                    return false;
                }
                $('#lblUserId').text(ui.item.val);
            }
        });
    }