jquery.min.js:2 Uncaught TypeError: Cannot use 'in' operator to search for 'length' in error
    at d (jquery.min.js:2)
    at Function.each (jquery.min.js:2)
    at Object.success (MostrarMascotas:485)
    at c (jquery.min.js:2)
    at Object.fireWith [as resolveWith] (jquery.min.js:2)
    at l (jquery.min.js:2)
    at XMLHttpRequest.<anonymous> (jquery.min.js:2)

    d @ jquery.min.js:2
    each @ jquery.min.js:2
    success @ MostrarMascotas:485
    c @ jquery.min.js:2
    fireWith @ jquery.min.js:2
    l @ jquery.min.js:2
    (anonymous) @ jquery.min.js:2

$.ajax({
            type: "POST",
            url: '@Url.Action("BusquedaInstantanea", "Pruebas")',
            data: JSON.stringify(array_mascotas),
            dataType: "json",
            contentType: "application/json; charset=utf-8",
            success: function (rpta) {

                $(".tr-mascota").remove();

                $.each(rpta, function (index, element) {

                    var registro =

                        '<tr class="tr-mascota">'+
                            '<td class="id_mascota">'+
                                element.id_mascota+
                            '</td>'+
                            '<td class="dni_mascota">'+
                                element.dni_mascota+
                            '</td>'+
                            '<td class="nombre_mascota">'+
                                element.nombre_mascota+
                            '</td>'+
                            '<td class="ape_pat_mascota">'+
                                element.ape_pat_mascota+
                            '</td>'+
                            '<td class="ape_mat_mascota">'+
                                element.ape_mat_mascota+
                            '</td>'+
                            '<td>'+
                                '@Html.ActionLink("Edit", "Edit", new { /* id=item.PrimaryKey */ })' + '|' +
                                '@Html.ActionLink("Details", "Details", new { /* id=item.PrimaryKey */ })' + '|' +
                                '@Html.ActionLink("Delete", "Delete", new { /* id=item.PrimaryKey */ })' +
                            '</td>'+
                        '</tr>';

                    $("tbody").append(registro);
                });

            },
            error: function (req, textStatus, errorThrown) {
                alert('Ooops, something happened: ' + textStatus + ' ' + errorThrown);
            }
        });

[HttpPost]
    public JsonResult BusquedaInstantanea(string data)
    {
        List<ClsMascota> temporal = new List<ClsMascota>();

        try
        {
            SqlCommand cmd = new SqlCommand("USP_SEARCH_LIVE", con);
            cmd.CommandType = CommandType.StoredProcedure;
            cmd.Parameters.AddWithValue("@data", data);
            con.Open();

            SqlDataReader read = cmd.ExecuteReader();
            while (read.Read())
            {
                ClsMascota mascota = new ClsMascota()
                {
                    id_mascota = read.GetInt32(0).ToString(),
                    dni_mascota = read.GetString(1),
                    nombre_mascota = read.GetString(2),
                    ape_pat_mascota = read.GetString(3),
                    ape_mat_mascota = read.GetString(4)
                };

                temporal.Add(mascota);
            }
            read.Close();
        }
        catch
        {
            return Json("error");
        }
        finally
        {

            con.Close();
        }

        return Json(temporal);
    }