diff --git a/Gemfile b/Gemfile
index 11eeabbe..096efc3c 100644
--- a/Gemfile
+++ b/Gemfile
@@ -143,6 +143,9 @@ gem 'country_select'
 # as PDF generator
 gem 'prawn-qrcode'
 gem 'prawn-rails'
+# FIXME: for prawn, matrix isn't in the default set of Ruby 3.1 anymore
+# see https://github.com/prawnpdf/prawn/commit/3658d5125c3b20eb11484c3b039ca6b89dc7d1b7
+gem 'matrix', '~> 0.4'
 
 # for QR code generation
 gem 'rqrcode'
diff --git a/Gemfile.lock b/Gemfile.lock
index 8267d4c3..2fae724f 100644
--- a/Gemfile.lock
+++ b/Gemfile.lock
@@ -298,6 +298,7 @@ GEM
     mail (2.7.1)
       mini_mime (>= 0.1.1)
     marcel (1.0.1)
+    matrix (0.4.2)
     method_source (1.0.0)
     mime-types (3.3.1)
       mime-types-data (~> 3.2015)
@@ -662,6 +663,7 @@ DEPENDENCIES
   leaflet-rails
   letter_opener
   letter_opener_web
+  matrix (~> 0.4)
   mina
   mini_magick
   money-rails
diff --git a/Gemfile.next.lock b/Gemfile.next.lock
index e34b6636..9d0d8526 100644
--- a/Gemfile.next.lock
+++ b/Gemfile.next.lock
@@ -324,6 +324,7 @@ GEM
     mail (2.7.1)
       mini_mime (>= 0.1.1)
     marcel (1.0.1)
+    matrix (0.4.2)
     method_source (1.0.0)
     mime-types (3.3.1)
       mime-types-data (~> 3.2015)
@@ -699,6 +700,7 @@ DEPENDENCIES
   leaflet-rails
   letter_opener
   letter_opener_web
+  matrix (~> 0.4)
   mina
   mini_magick
   money-rails