Wrong type argument: integerp, nil in VM 6.76 under GNU Emacs 20.5.1

[Eduardo Sontag]

This bug report will be sent to the Free Software Foundation,
 not to your local site managers!!
Please write in English, because the Emacs maintainers do not have
translators to read other languages for them.

In GNU Emacs 20.5.1 (i386-redhat-linux-gnu, X toolkit)
 of Mon Feb 21 2000 on porky.devel.redhat.com
configured using `configure  --prefix=/usr --libexecdir=/usr/lib 
--sharedstatedir=/var --with-gcc --with-pop --with-x-toolkit 
i386-redhat-linux-gnu'

Please describe exactly what actions triggered the bug
and the precise symptoms of the bug:

===========================================================================
Using VM 6.76, after having tried to save ("write") a message which contains
the string shown below, I start receiving the following message repeatedly:

   Wrong type argument: integerp, nil

whenever executing any command.
I determined that this string is the offender by cutting selected parts of the
message (editing INBOX outside VM) and restarting VM after each such
experiment.
==========================================================================
The string is as follows.  I have replaced a name by the string NAME and an
email address by the string EMAIL for privacy reasons.
I believe that the problem is the "n~" in Spanish!

X-VM-v5-Data: ([nil nil nil nil nil nil nil nil nil]
        ["1337" "Friday" "1" "December" "2000" "17:11:54" "-0300" "NAME" 
"EMAIL@fibertel.com.ar" nil "43" "=?iso-8859-1?Q?Espa=F1a?=" "^From:" nil nil 
"12" nil nil (number " " mark "     NAME Dec  1   43/1337  " thread-indent 
"\"España\"\n") nil nil]
        nil)