[EXIM] expansion contains non-printing characters?

Hello,

I have the following transport:

t_helpdesk:
  driver                = autoreply
  from                  = "$sender_address"
  to                    = "helpdesk@???"
  subject               = "${uc:$original_local_part}"
  text                  = "$header_subject"
  user                  = exim
  group                 = exim
  return_message        = true

This transport encapsulates the original email and sends it to another
account, where brain dead NT helpdesk software runs.

I got the following error message:

    Expansion of "$header_subject" in t_helpdesk transport contains non-printing characters

It looks like a user might have sent a mail with a special character in the
subject. Is there any way to fix that problem besides removing text = ?

Michael

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