421 4.3.0 deliver: Trying to unput wrong character

My procmail is configured to feed incoming mail into deliver:

    :0wr
    |sed 1d | /opt/cyrus/bin/deliver mi

Recently I received an e-mail consisting of a PNG-file -- not an 
attachment, the body of the e-mail had content-type image/png. That 
message ended up in my /var/mail/mi mailbox -- because deliver would not 
accept it:

    421 4.3.0 deliver: Trying to unput wrong character

Trying to upload the same message using mbox2imap, I get:

    mbox2imap: (alert) IMAP protocol error
    mbox2imap: (alert) Protocol response: NO
    mbox2imap: (alert) Diagnostic message: ['Message contains NUL
    characters']

Why is deliver saying this, and what can I do to be able to receive such 
mail through Cyrus the way I was able to receive them through regular 
Unix mail for years? Thank you!

    -mi

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