Callback in Netrender

I'm trying to Setcallback to netrender like this:

m = NetRender.GetManager()
m.connect #manual "renderer1" --connecting manager
if m.QueryControl #dontwait do--checking whether its free
    m.GetControl() --if free than taking control over it
    m.Lock(true) --disabling others to connect to manager
    job = m.newJob() --submitting the current scene

    fn myupdate =
        MessageBox "Manager updated "

    m.setcallback #update myupdate = "aTest" --naming job
    job.IncludeMaps = true --maps must be included
    job.FromFrame = 0 --render Single Frame
    job.toFrame = 0  --render Single Frame
    job.submit() --sending job to server

Job submits, but message box doesn't appear, and Max hungs up. I wonder whats the problem is, it seems all correct. Help!