Bad substitution -- passing environment variables to bash

I'm getting this in my log file:

procmail: Assigning "NEWHEAD="
procmail: Bad substitution of "RESENDMAIL_O"
procmail: Executing "formail,-a,Message-ID:,-a,Folder: /home/robertb/}"

The code is:

        :0hw
        NEWHEAD=|formail -a Message-ID: -a "Folder: "${RESENDMAIL_O#$HOME/}

($RESENDMAIL_O happens to have the value "/home/robertb", but that
shouldn't matter.)

I think procmail is trying to substitute the environment variable _before_
the shell is executed.  Because of this, I'm getting a screwed-up value
("/home/robertb/}" is wrong).  When I do this formail command in bash
directly, it works perfectly.

Is there a way to disable procmail from doing any substitutions when executing
something in the context of the shell?

% procmail -v
procmail v3.13.1 1999/04/05, Copyright (c) 1999, Stephen R. van den Berg
                                                        <srb(_at_)cuci(_dot_)nl>

Locking strategies:     dotlocking, fcntl()
Default rcfile:         $HOME/.procmailrc
Your system mailbox:    /var/spool/mail/robertb

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