Failed to verify a custom form published in the personal forms library

[Bingo]

I'm using the following code to create a mail item with a
custom form. Then I check the messageclass property of
this newly created mail item to find out if the custom
form exists. If the form is published in the
organizational forms library, it works fine. But if it's
published in the personal forms library, the message
class of the new mail item is always the default,
IPM.Note. What's the problem? Thanks.

Set mapiMailItem = mapiDraftFolder.Items.Add(messageClass)

mapiMailItem.messageClass