Uncaught TypeError: Cannot read property 'length' of undefined

<script>
...
...
    .done(function (data) {
        if (data) {
            $.each(data.responseJSON, function (key, value) {
                var span = '#formCompany span[id=span-' + key + ']';
                $(span + '>strong').text(value);
                $(span).parent().parent().addClass('has-error');
            });
        }
        else {
            $(".alert-success").prop("hidden", false);
        }
    })
            .fail(function (data) {
                alert('The server failed somewhere');
            });

</script>

{
"title":["The title field is required."],
"content":["The content field is required."],
...
}