[web2py] crud.update returning 404 NOT FOUND ??

[Luca]

I have the following simple code, which is meant to let a user do a new 
submission, or update a submission if one exists:

    sub = db(db.submission.author == auth.user_id).select().first() 
    form = crud.update(db, 
        record=sub,
        deletable=True,
        next=URL('default', 'index'),
        )
    return dict(form=form)

sub is None, initially, as there is no submission, and so the crud (I am 
hoping) should behave like a create. 
Instead, I get a 404 NOT FOUND.  Is this normal?  What's the proper way of 
doing this? 

It's  a bit bizarre also to get a page that says 404 NOT FOUND, rather than 
having my browser report 404 because it is not finding a page. 
It's like getting a letter in the mail that says, today you got no mail! 
:-) 

Thanks for all the help, 

Luca

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