Sieve runtime error "mailbox does not exist"

Hello again!

Pushing my luck asking a second question so soon, sorry.  This is the last step before I can declare a mailserver to be configured, and ready for users to break it.  (Users should not be allowed; they spoil everything.)

I am getting this when running a sieve script:
www cyrus/lmtpunix[4215]: sieve runtime error for [email protected] id <CALvLiS=...>: Fileinto (Spam): Mailbox does not exist

So it is clear the sieve script is being picked up and processed.  It is absurdly simple:
require "fileinto";
if header :contains "X-Spam-Flag" "YES" {
    fileinto "Spam";
}

cyradm tells me that the Spam folder is there:
localhost> lm
user/test.three/[email protected] (\HasNoChildren)
user/test.three/[email protected] (\HasNoChildren)
user/test.three/[email protected] (\HasNoChildren)
user/test.three/[email protected] (\HasNoChildren)
user/test.three/[email protected] (\HasNoChildren)
user/[email protected] (\HasNoChildren)

These are being created automatically, courtesy of:
autocreate_quota: 0
autocreate_inbox_folders: Sent | Trash | Spam | Drafts | Deleted
autocreate_subscribe_folders: Sent | Spam

... in imapd.conf.

Mail is being delivered to the inbox quite happily - it's just the sieve which is failing.

I'm not hugely confident with sieve configuration, so I imagine this is another silly configuration error I've overlooked.  Any pointing-out of the glaringly obvious will be gratefully received.


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