diff --git a/app/mailers/notify.rb b/app/mailers/notify.rb
index 662f3e00047..88ad4c3e893 100644
--- a/app/mailers/notify.rb
+++ b/app/mailers/notify.rb
@@ -166,7 +166,7 @@ class Notify < BaseMailer
     headers['In-Reply-To'] = message_id(model)
     headers['References'] = [message_id(model)]
 
-    headers[:subject]&.prepend('Re: ')
+    headers[:subject] = "Re: #{headers[:subject]}" if headers[:subject]
 
     mail_thread(model, headers)
   end
diff --git a/changelogs/unreleased/55104-frozenerror-can-t-modify-frozen-string.yml b/changelogs/unreleased/55104-frozenerror-can-t-modify-frozen-string.yml
new file mode 100644
index 00000000000..994859b1d1d
--- /dev/null
+++ b/changelogs/unreleased/55104-frozenerror-can-t-modify-frozen-string.yml
@@ -0,0 +1,5 @@
+---
+title: Fix a frozen string error in app/mailers/notify.rb
+merge_request: 23683
+author:
+type: fixed
diff --git a/spec/mailers/notify_spec.rb b/spec/mailers/notify_spec.rb
index 150c00e4bfe..1d17aec0ded 100644
--- a/spec/mailers/notify_spec.rb
+++ b/spec/mailers/notify_spec.rb
@@ -28,8 +28,8 @@ describe Notify do
   end
 
   def have_referable_subject(referable, reply: false)
-    prefix = referable.project ? "#{referable.project.name} | " : ''
-    prefix.prepend('Re: ') if reply
+    prefix = (referable.project ? "#{referable.project.name} | " : '').freeze
+    prefix = "Re: #{prefix}" if reply
 
     suffix = "#{referable.title} (#{referable.to_reference})"