Closing bootstrap modal from asp.net code-behind

I found this answer, but get the following error:

script:

<script type="text/javascript">
    function Closepopup() {
        debugger;
        $('#myModal').modal('close');
    }
</script>

code-behind:

    ScriptManager.RegisterStartupScript(Me, Me.GetType, "Close Modal Popup", "Closepopup();", True)

error:

Unhandled exception at line 6, column 13594 in http://localhost:1230/Scripts/bootstrap.min.js

0x800a01b6 - JavaScript runtime error: Object doesn't support property or method 'close'

Any ideas appreciated!

modal('toggle') instead of modal(toggle)

$(function () {
   $('#modal').modal('toggle');
});