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

m'>log</a><a href='/guix/tree/tests/union.scm?id=c5c555b186a894e6bd3d5709c5199fcab1f0b7d0'>tree</a><a href='/guix/commit/tests/union.scm?id=c5c555b186a894e6bd3d5709c5199fcab1f0b7d0'>commit</a><a href='/guix/diff/tests/union.scm?id=c5c555b186a894e6bd3d5709c5199fcab1f0b7d0'>diff</a></td><td class='form'><form class='right' method='get' action='/guix/log/tests/union.scm'>
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<div class='path'>path: <a href='/guix/log/?id=c5c555b186a894e6bd3d5709c5199fcab1f0b7d0'>root</a>/<a href='/guix/log/tests?id=c5c555b186a894e6bd3d5709c5199fcab1f0b7d0'>tests</a>/<a href='/guix/log/tests/union.scm?id=c5c555b186a894e6bd3d5709c5199fcab1f0b7d0'>union.scm</a></div><div class='content'><table class='list nowrap'><tr class='nohover'><th class='left'>Age</th><th class='left'>Commit message (<a href='/guix/log/tests/union.scm?id=c5c555b186a894e6bd3d5709c5199fcab1f0b7d0&amp;showmsg=1'>Expand</a>)</th><th class='left'>Author</th></tr>
<tr><td><span title='2013-02-06 23:04:10 +0100'>2013-02-06</span></td><td><a href='/guix/commit/tests/union.scm?id=6211223021cdb477e93d7ba789d5a0119ffb4bd4'>union: Delete duplicates when passed the same input several times.</a><span class='msg-avail'>...</span></td><td>Ludovic Courtès</td></tr>
<tr><td><span title='2013-02-01 13:16:45 +0100'>2013-02-01</span></td><td><a href='/guix/commit/tests/union.scm?id=0bdba772ca2167cdd3acaed81ca8667cb78b266c'>Augment `.dir-locals.el'.</a><span class='msg-avail'>...</span></td><td>Ludovic Courtès</td></tr>
<tr><td><span title='2013-01-18 01:07:31 +0100'>2013-01-18</span></td><td><a href='/guix/commit/tests/union.scm?id=1ffa7090b99dfd2f54fa883929c5e78d7852657a'>distro: Change the module name space to (gnu ...).</a><span class='msg-avail'>...</span></td><td>Ludovic Courtès</td></tr>
<tr><td><span title='2013-01-11 16:01:49 +0100'>2013-01-11</span></td><td><a href='/guix/commit/tests/union.scm?id=01e354eb83299d00ddd6ba4beb73bac8130beeae'>Merge branch 'core-updates'</a><span class='msg-avail'>...</span></td><td>Ludovic Courtès</td></tr>
<tr><td><span title='2013-01-09 22:10:06 +0100'>2013-01-09</span></td><td><a href='/guix/commit/tests/union.scm?id=b2d58cd80a04ccab09a947d187ae55ff199eae08'>union: Detect collisions, and delete duplicate leaves.</a><span class='msg-avail'>...</span></td><td>Ludovic Courtès</td></tr>
<tr><td><span title='2013-01-06 17:33:02 +0100'>2013-01-06</span></td><td><a href='/guix/commit/tests/union.scm?id=4050e5d6cfe8f7af29f10b2f1b3c7febdc10946a'>Merge branch 'master' into core-updates</a><span class='msg-avail'>...</span></td><td>Ludovic Courtès</td></tr>
<tr><td><span title='2013-01-06 00:47:50 +0100'>2013-01-06</span></td><td><a href='/guix/commit/tests/union.scm?id=233e76769ae3a438bff7117c68f2c88739a28db0'>Update license headers.</a><span class='msg-avail'>...</span></td><td>Ludovic Courtès</td></tr>
<tr><td><span title='2013-01-02 01:04:37 +0100'>2013-01-02</span></td><td><a href='/guix/commit/tests/union.scm?id=97d3998e7a8a8e22fbef2e42db66456b826b1695'>tests: Adjust to not rely on /bin/sh.</a><span class='msg-avail'>...</span></td><td>Ludovic Courtès</td></tr>
<tr><td><span title='2012-12-13 22:06:45 +0100'>2012-12-13</span></td><td><a href='/guix/commit/tests/union.scm?id=ad1ebab379dbd68e006197e8609c67de4734dbde'>tests: Skip network-dependent tests when the network is unreachable.</a><span class='msg-avail'>...</span></td><td>Ludovic Courtès</td></tr>
<tr><td><span title='2012-11-04 22:04:41 +0100'>2012-11-04</span></td><td><a href='/guix/commit/tests/union.scm?id=18633d4f350160c2eb6f5101a628010322336e91'>distro: Move bootstrap packages to (distro packages bootstrap).</a><span class='msg-avail'>...</span></td><td>Ludovic Courtès</td></tr>
<tr><td><span title='2012-10-29 22:39:46 +0100'>2012-10-29</span></td><td><a href='/guix/commit/tests/union.scm?id=c8c88afaa14f760ec71ffb2ef0d712b7e42cc71f'>Add (guix build union).</a><span class='msg-avail'>...</span></td><td>Ludovic Courtès</td></tr>
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