Returning custom autoexpunge for user's INBOX

I want to be able to have custom autoexpunge settings for different users.

I have it working for various folders (draft, junk, trash), but I can't seem to get it working for the user's primary inbox.

In my user_query (I'm using mysql), I'm returning entries like this in the userdb_import field:

   namespace/inbox/mailbox/autoexpunge=5d	namespace/inbox/mailbox/Drafts/autoexpunge=5d	namespace/inbox/mailbox/Junk/autoexpunge=5d	namespace/inbox/mailbox/Trash/autoexpunge=5d

And everything works fine for Drafts, Junk, and Trash, but not for the main inbox. The logs show:

Debug: Unknown userdb setting: plugin/namespace/inbox/mailbox/autoexpunge=5d

I've also tried variations, trying to get the namespace correct for the main inbox, but they all get the same "Unknown userdb setting" error:

namespace/inbox/mailbox/autoexpunge=5d
namespace/inbox/mailbox/inbox/autoexpunge=5d
namespace/inbox/mailbox/Inbox/autoexpunge=5d
namespace/inbox/mailbox/INBOX/autoexpunge=5d

I suspect I'm just giving the wrong namespace for the primary inbox.

What's the right way to do this?

Tom