This patch has been proposed upstream:
https://lists.systemreboot.net/guile-email/87mtnv1r2p.fsf@gnu.org/T/#u.

From c21fe0f0e28b80b606973d3e372e2bc8528c9766 Mon Sep 17 00:00:00 2001
From: Mathieu Othacehe <othacehe@gnu.org>
Date: Wed, 29 Sep 2021 12:47:35 +0000
Subject: [PATCH 1/1] email: Do not use an empty bytevector to test the
 charset.

Using an empty bytevector no longer throws an exception since this Guile
commit: 5ea8c69e9153a970952bf6f0b32c4fad6a28e839.

* email/email.scm (post-process-content-transfer-encoding): Use a bytevector
containg the 'e' character to test the charset validity.
---
 email/email.scm | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/email/email.scm b/email/email.scm
index 3f4e194..ac70463 100644
--- a/email/email.scm
+++ b/email/email.scm
@@ -832,7 +832,8 @@ values. The returned headers is a string and body is a bytevector."
   (define (valid-charset? charset)
     (catch #t
       (lambda ()
-        (bytevector->string (make-bytevector 0 0) charset)
+        ;; Try to convert a bytevector containg the 'e' character.
+        (bytevector->string (make-bytevector 1 48) charset)
         #t)
       (const #f)))
 
-- 
2.33.0

n.ac'>log</a><a href='/guix/tree/config-daemon.ac?id=b8b149eb37718aea4f80aa47a3d1cd1854278abf'>tree</a><a href='/guix/commit/config-daemon.ac?id=b8b149eb37718aea4f80aa47a3d1cd1854278abf'>commit</a><a href='/guix/diff/config-daemon.ac?id=b8b149eb37718aea4f80aa47a3d1cd1854278abf'>diff</a></td><td class='form'><form class='right' method='get' action='/guix/log/config-daemon.ac'>
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<tr><td><span title='2021-05-03 09:29:44 -0400'>2021-05-03</span></td><td><a href='/guix/commit/po/packages/Makevars?id=a23789bbbdd23b1130f0f2521ee8d68872b7fc2c'>nls: Do not update po files on first make invocation.</a><span class='msg-avail'>...<span class='msg-tooltip'>We need to update the minimal gettext version to take advantage of new
features.  Before this patch, the first make invocation would modify
po/guix and po/packages po files, and we advised to run `git checkout
po` to clean the changes.

* configure.ac (AM_GNU_GETTEXT_VERSION): Update to 0.19.1.
* po/guix/Makevars: Set PO_DEPENDS_ON_POT to no.
* po/packages/Makevars: Set PO_DEPENDS_ON_POT to no.

Signed-off-by: Maxim Cournoyer &lt;maxim.cournoyer@gmail.com&gt;
</span></span></td><td>Julien Lepiller</td></tr>
<tr><td><span title='2020-09-29 15:40:25 +0200'>2020-09-29</span></td><td><a href='/guix/commit/po/packages/Makevars?id=ac324be105977d531928ef7da79483ae7a461b68'>nls: Fix copyright and bug address in pot files.</a><span class='msg-avail'>...<span class='msg-tooltip'>* po/guix/Makevars: Fix COPYRIGHT_HOLDER and MSGID_BUGS_ADDRESS.
* po/packages/Makevars: idem.
</span></span></td><td>Julien Lepiller</td></tr>