diff --git a/.github/workflows/ruby.yml b/.github/workflows/ruby.yml
index ef4d9c26..35ab0f85 100644
--- a/.github/workflows/ruby.yml
+++ b/.github/workflows/ruby.yml
@@ -17,6 +17,7 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
+        rubyopt: [""]
         ruby_version: [head, 3.4, 3.3, 3.2, jruby]
         gemfile:
           - Gemfile
@@ -34,6 +35,10 @@ jobs:
             gemfile: gemfiles/Gemfile.rails-8.0.x
           - ruby_version: jruby
             gemfile: gemfiles/Gemfile.rails-main
+        include:
+          - ruby_version: 3.4
+            gemfile: Gemfile
+            rubyopt: "--enable-frozen-string-literal --debug-frozen-string-literal"
 
     runs-on: ubuntu-latest
 
diff --git a/lib/i18n/backend/interpolation_compiler.rb b/lib/i18n/backend/interpolation_compiler.rb
index e37b6799..0e5c8727 100644
--- a/lib/i18n/backend/interpolation_compiler.rb
+++ b/lib/i18n/backend/interpolation_compiler.rb
@@ -25,6 +25,7 @@ module Compiler
 
         def compile_if_an_interpolation(string)
           if interpolated_str?(string)
+            string = +string
             string.instance_eval <<-RUBY_EVAL, __FILE__, __LINE__
               def i18n_interpolate(v = {})
                 "#{compiled_interpolation_body(string)}"
diff --git a/lib/i18n/tests/interpolation.rb b/lib/i18n/tests/interpolation.rb
index 03c67db7..991c4ecd 100644
--- a/lib/i18n/tests/interpolation.rb
+++ b/lib/i18n/tests/interpolation.rb
@@ -89,14 +89,14 @@ module Interpolation
         end
 
         test "interpolation: ASCII strings in the backend should be encoded to UTF8 if interpolation options are in UTF8" do
-          I18n.backend.store_translations 'en', 'encoding' => ('%{who} let me go'.force_encoding("ASCII"))
+          I18n.backend.store_translations 'en', 'encoding' => ('%{who} let me go'.dup.force_encoding(Encoding::US_ASCII))
           result = I18n.t 'encoding', :who => "måmmå miå"
           assert_equal Encoding::UTF_8, result.encoding
         end
 
         test "interpolation: UTF8 strings in the backend are still returned as UTF8 with ASCII interpolation" do
           I18n.backend.store_translations 'en', 'encoding' => 'måmmå miå %{what}'
-          result = I18n.t 'encoding', :what => 'let me go'.force_encoding("ASCII")
+          result = I18n.t 'encoding', :what => 'let me go'.dup.force_encoding(Encoding::US_ASCII)
           assert_equal Encoding::UTF_8, result.encoding
         end
 
@@ -172,7 +172,7 @@ def capture(stream)
       end
 
       def euc_jp(string)
-        string.encode!(Encoding::EUC_JP)
+        string.encode(Encoding::EUC_JP)
       end
 
       def interpolate(*args)
diff --git a/test/backend/metadata_test.rb b/test/backend/metadata_test.rb
index 44612cf0..84507c0d 100644
--- a/test/backend/metadata_test.rb
+++ b/test/backend/metadata_test.rb
@@ -38,7 +38,7 @@ def setup
   end
 
   test "pluralization adds the count to metadata on Strings" do
-    assert_equal(1, I18n.t(:missing, :count => 1, :default => { :one => 'foo' }).translation_metadata[:count])
+    assert_equal(1, I18n.t(:missing, :count => 1, :default => { :one => +'foo' }).translation_metadata[:count])
   end
 
   test "metadata works with frozen values" do