diff --git a/CHANGELOG.md b/CHANGELOG.md
index c4e35d4..f0668f7 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -1,6 +1,7 @@
 ## [Unreleased]
 ### Fixed
 * Support Ruby 3.4, Rails 8.0
+* Fix ruby 3.4 frozen string literal warnings
 
 ## 2.4.2 / 2025-03-23
 * fix link with icon button style
diff --git a/app/helpers/ndr_error/errors_helper.rb b/app/helpers/ndr_error/errors_helper.rb
index 79ded8a..aad9433 100644
--- a/app/helpers/ndr_error/errors_helper.rb
+++ b/app/helpers/ndr_error/errors_helper.rb
@@ -11,7 +11,7 @@ def highlighted_trace_for(error)
 
       error.backtrace.map do |line|
         css_classes = 'trace-item'
-        css_classes << ' stack-only' unless highlighting.include?(line)
+        css_classes += ' stack-only' unless highlighting.include?(line)
 
         content_tag(:span, line, class: css_classes)
       end
@@ -59,7 +59,7 @@ def downstream_fingerprint_link(print)
     def ticket_link_for(fingerprint, small = false) # rubocop:disable Style/OptionalBooleanParameter
       text = bootstrap_icon_tag('asterisk', :bi) + ' View ticket' # rubocop:disable Style/StringConcatenation
       css  = 'btn btn-outline-secondary'
-      css << ' btn-xs' if small
+      css += ' btn-xs' if small
 
       url = fingerprint.ticket_url
       link_to(text, /^http/i =~ url ? url : "http://#{url}", class: css)
@@ -87,7 +87,7 @@ def purge_button_for(fingerprint)
 
     def previous_button_for(error)
       css = 'btn btn-outline-secondary'
-      css << ' disabled' if error.nil?
+      css += ' disabled' if error.nil?
       text = bootstrap_icon_tag('chevron-left', :bi)
       path = error.nil? ? '#' : error_fingerprint_path(error.error_fingerprint, log_id: error)
 
@@ -96,7 +96,7 @@ def previous_button_for(error)
 
     def next_button_for(error)
       css = 'btn btn-outline-secondary'
-      css << ' disabled' if error.nil?
+      css += ' disabled' if error.nil?
       text = bootstrap_icon_tag('chevron-right', :bi)
       path = error.nil? ? '#' : error_fingerprint_path(error.error_fingerprint, log_id: error)
 
diff --git a/ndr_error.gemspec b/ndr_error.gemspec
index ecc62c2..3eb4556 100644
--- a/ndr_error.gemspec
+++ b/ndr_error.gemspec
@@ -45,6 +45,13 @@ Gem::Specification.new do |s|
   # https://bugs.ruby-lang.org/issues/19371
   s.add_development_dependency 'psych', '< 5'
 
+  # Workaround build issue on GitHub Actions with ruby <= 3.1 when installing sass-embedded
+  # Versions 1.77.0 and above require ruby >= 3.2 for packaged x86_64-linux binaries
+  # but say they're compatible with ruby 3.1.
+  # 1.76.0 has a bug that was fixed in 1.77.1, which we can't use yet.
+  # https://github.com/sass/dart-sass/issues/2239
+  s.add_development_dependency 'sass-embedded', '~> 1.75.0' if RUBY_VERSION.start_with?('3.1') # rubocop:disable Gemspec/RubyVersionGlobalsUsage
+
   s.add_development_dependency 'mocha'
   s.add_development_dependency 'test-unit', '~> 3.0'