diff --git a/.gitlab-ci.yml b/.gitlab-ci.yml
index ec56f23..ad13b8c 100644
--- a/.gitlab-ci.yml
+++ b/.gitlab-ci.yml
@@ -51,7 +51,8 @@ schleuder:
   script:
     - debconf-set-selections <<<"postfix postfix/mailname string example.org"
     - debconf-set-selections <<<"postfix postfix/main_mailer_type string 'Local only'"
-    - apt update; apt install --yes schleuder schleuder-cli postfix
+    - apt update; apt install --yes schleuder schleuder-cli postfix patch
+    - patch -p1 < test/01-ruby3cgi.patch  # https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=1040257
     - /usr/lib/postfix/configure-instance.sh -
     - echo "virtual_alias_maps = static:root" >> /etc/postfix/main.cf
     - /usr/sbin/postmulti -i - -p start
diff --git a/test/01-ruby3cgi.patch b/test/01-ruby3cgi.patch
new file mode 100644
index 0000000..25bc897
--- /dev/null
+++ b/test/01-ruby3cgi.patch
@@ -0,0 +1,21 @@
+--- a/usr/lib/ruby/vendor_ruby/schleuder-cli.rb	2023-07-04 01:00:03.032907109 +0000
++++ b/usr/lib/ruby/vendor_ruby/schleuder-cli.rb	2023-07-04 00:59:05.299382428 +0000
+@@ -3,6 +3,7 @@
+ require 'pathname'
+ require 'net/https'
+ require 'uri'
++require 'cgi'
+ require 'singleton'
+ require 'yaml'
+ require 'base64'
+--- a/usr/lib/ruby/vendor_ruby/schleuder-cli/helper.rb	2023-07-04 00:59:55.332884367 +0000
++++ b/usr/lib/ruby/vendor_ruby/schleuder-cli/helper.rb	2023-07-04 00:59:20.859439658 +0000
+@@ -19,7 +19,7 @@
+       u = "/#{args.join('/')}.json"
+       if params
+         paramstring  = params.map do |k,v|
+-          "#{URI.escape(k.to_s)}=#{URI.escape(v.to_s)}"
++          "#{CGI.escape(k.to_s)}=#{CGI.escape(v.to_s)}"
+         end.join('&')
+         u << "?#{paramstring}"
+       end