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

0c0e7cb2faee5b394'>refs</a><a class='active' href='/guix/log/etc/snippets/scheme-mode?showmsg=1'>log</a><a href='/guix/tree/etc/snippets/scheme-mode?id=e1485cd9ad9520b1e788f9f0c0e7cb2faee5b394'>tree</a><a href='/guix/commit/etc/snippets/scheme-mode?id=e1485cd9ad9520b1e788f9f0c0e7cb2faee5b394'>commit</a><a href='/guix/diff/etc/snippets/scheme-mode?id=e1485cd9ad9520b1e788f9f0c0e7cb2faee5b394'>diff</a></td><td class='form'><form class='right' method='get' action='/guix/log/etc/snippets/scheme-mode'>
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<div class='path'>path: <a href='/guix/log/?id=e1485cd9ad9520b1e788f9f0c0e7cb2faee5b394&amp;showmsg=1'>root</a>/<a href='/guix/log/etc?id=e1485cd9ad9520b1e788f9f0c0e7cb2faee5b394&amp;showmsg=1'>etc</a>/<a href='/guix/log/etc/snippets?id=e1485cd9ad9520b1e788f9f0c0e7cb2faee5b394&amp;showmsg=1'>snippets</a>/<a href='/guix/log/etc/snippets/scheme-mode?id=e1485cd9ad9520b1e788f9f0c0e7cb2faee5b394&amp;showmsg=1'>scheme-mode</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/etc/snippets/scheme-mode?id=e1485cd9ad9520b1e788f9f0c0e7cb2faee5b394'>Collapse</a>)</th><th class='left'>Author</th></tr>
<tr class='logheader'><td><span title='2019-05-09 00:22:37 -0400'>2019-05-09</span></td><td class='logsubject'><a href='/guix/commit/etc/snippets/scheme-mode?id=4ac69ea10f54384ac893313ef738e5ad554ac328'>Add (guix bzr-download).</a></td><td>Maxim Cournoyer</td></tr>
<tr class='nohover-highlight'><td/><td colspan='3' class='logmsg'>
* guix/bzr-download.scm, guix/build/bzr.scm,
etc/snippets/scheme-mode/guix-bzr-reference: New files.
* Makefile.am (MODULES): Add them.
* etc/snippets/scheme-mode/guix-origin: Add "bzr-fetch" to the origin choices.


</td></tr>
af4733.

* guix.scm: Use #:re-export-and-replace for 'define-public'.


</td></tr>
<tr class='logheader'><td><span title='2021-06-29 00:00:13 +0200'>2021-06-29</span></td><td class='logsubject'><a href='/guix/commit/guix.scm?id=e059adcb6001ab0353d969b24030d0c342f8830e'>(guix) no longer pulls in (guix ftp-client).</a></td><td>Ludovic Courtès</td></tr>
<tr class='nohover-highlight'><td/><td colspan='3' class='logmsg'>
That choice was made in 2014 and never made much sense.

* guix.scm (%public-modules): Remove 'ftp-client'.


</td></tr>
/a><span class='msg-avail'>...<span class='msg-tooltip'>* gnu/packages/node-xyz.scm (node-oop): New variable.
</span></span></td><td>Efraim Flashner</td></tr>
<tr><td><span title='2020-01-20 09:39:54 +0200'>2020-01-20</span></td><td><a href='/guix/commit/gnu/packages/node-xyz.scm?id=d1095c47f1bedcc10f8bc10d80b7f2de3ad8989b'>gnu: Add node-util-deprecate.</a><span class='msg-avail'>...<span class='msg-tooltip'>* gnu/packages/node-xyz.scm (node-util-deprecate): New variable.
</span></span></td><td>Efraim Flashner</td></tr>
<tr><td><span title='2020-01-20 09:39:53 +0200'>2020-01-20</span></td><td><a href='/guix/commit/gnu/packages/node-xyz.scm?id=c8bbff7ee13b6c2b1a97edcf530685216955b471'>gnu: Add node-statsd-parser.</a><span class='msg-avail'>...<span class='msg-tooltip'>* gnu/packages/node-xyz.scm (node-statsd-parser): New variable.
</span></span></td><td>Efraim Flashner</td></tr>
<tr><td><span title='2020-01-20 09:39:51 +0200'>2020-01-20</span></td><td><a href='/guix/commit/gnu/packages/node-xyz.scm?id=a971890b1da4543455d079b105364c390301986e'>gnu: Add node-mersenne.</a><span class='msg-avail'>...<span class='msg-tooltip'>* gnu/packages/node-xyz.scm (node-mersenne): New variable.
</span></span></td><td>Efraim Flashner</td></tr>
<tr><td><span title='2020-01-20 09:39:49 +0200'>2020-01-20</span></td><td><a href='/guix/commit/gnu/packages/node-xyz.scm?id=2f2e7be944c80d5fdb7fcab90ecf2ddf4b80c416'>gnu: Add node-long-stack-traces.</a><span class='msg-avail'>...<span class='msg-tooltip'>* gnu/packages/node-xyz.scm (node-long-stack-traces): New variable.
</span></span></td><td>Efraim Flashner</td></tr>
<tr><td><span title='2020-01-20 09:39:47 +0200'>2020-01-20</span></td><td><a href='/guix/commit/gnu/packages/node-xyz.scm?id=0a2d61a5944c24d18329b3534c06a3b11010b19d'>gnu: Add node-env-variable.</a><span class='msg-avail'>...<span class='msg-tooltip'>* gnu/packages/node-xyz.scm (node-env-variable): New variable.
* gnu/local.mk (GNU_SYSTEM_MODULES): Register it.
</span></span></td><td>Efraim Flashner</td></tr>