diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index 27f0ac8c..487ab6ce 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -30,6 +30,8 @@ jobs:
           bundler-cache: true
       - name: Run tests
         run: bundle exec rake test:units
+        env:
+          RUBYOPT: ${{ startsWith(matrix.ruby, 'head') && '--enable=frozen-string-literal' || '' }}
 
   test-legacy:
     runs-on: ubuntu-20.04
diff --git a/lib/sshkit/color.rb b/lib/sshkit/color.rb
index 91109874..74e2df9a 100644
--- a/lib/sshkit/color.rb
+++ b/lib/sshkit/color.rb
@@ -44,8 +44,8 @@ def colorize(obj, color, mode=nil)
       return string unless COLOR_CODES.key?(color)
 
       result = mode == :bold ? "\e[1;" : "\e[0;"
-      result << COLOR_CODES.fetch(color).to_s
-      result << ";49m#{string}\e[0m"
+
+      "#{result}#{COLOR_CODES.fetch(color)};49m#{string}\e[0m"
     end
 
     # Returns `true` if the underlying output is a tty, or if the SSHKIT_COLOR
diff --git a/lib/sshkit/deprecation_logger.rb b/lib/sshkit/deprecation_logger.rb
index bddc6c68..46101228 100644
--- a/lib/sshkit/deprecation_logger.rb
+++ b/lib/sshkit/deprecation_logger.rb
@@ -11,8 +11,14 @@ def log(message)
       return if @out.nil?
       warning_msg = "[Deprecated] #{message}\n"
       caller_line = caller.find { |line| !line.include?('lib/sshkit') }
-      warning_msg << "    (Called from #{caller_line})\n" unless caller_line.nil?
-      @out << warning_msg unless @previous_warnings.include?(warning_msg)
+      warning_msg = "#{warning_msg}    (Called from #{caller_line})\n" unless caller_line.nil?
+      unless @previous_warnings.include?(warning_msg)
+        if @out.frozen?
+          @out = "#{@out}#{warning_msg}"
+        else
+          @out << warning_msg
+        end
+      end
       @previous_warnings << warning_msg
     end
   end
diff --git a/test/unit/backends/test_abstract.rb b/test/unit/backends/test_abstract.rb
index 0ec8f8d7..0267cc02 100644
--- a/test/unit/backends/test_abstract.rb
+++ b/test/unit/backends/test_abstract.rb
@@ -112,14 +112,14 @@ def test_within_home
       end
 
       def test_background_logs_deprecation_warnings
-        deprecation_out = ''
+        deprecation_out = []
         SSHKit.config.deprecation_output = deprecation_out
 
         ExampleBackend.new do
           background :ls
         end.run
 
-        lines = deprecation_out.lines.to_a
+        lines = deprecation_out.join.lines.to_a
 
         assert_equal 2, lines.length
 
diff --git a/test/unit/test_command.rb b/test/unit/test_command.rb
index 30964a16..d5fe9d26 100644
--- a/test/unit/test_command.rb
+++ b/test/unit/test_command.rb
@@ -211,7 +211,7 @@ def test_on_stderr
     end
 
     def test_deprecated_stdtream_accessors
-      deprecation_out = ''
+      deprecation_out = []
       SSHKit.config.deprecation_output = deprecation_out
 
       c = Command.new(:whoami)
@@ -219,7 +219,7 @@ def test_deprecated_stdtream_accessors
       assert_equal('a test', c.stdout)
       c.stderr='another test'
       assert_equal('another test', c.stderr)
-      deprecation_lines = deprecation_out.lines.to_a
+      deprecation_lines = deprecation_out.join.lines.to_a
 
       assert_equal 8, deprecation_lines.size
       assert_equal(
diff --git a/test/unit/test_configuration.rb b/test/unit/test_configuration.rb
index e470daab..49a46fd2 100644
--- a/test/unit/test_configuration.rb
+++ b/test/unit/test_configuration.rb
@@ -11,10 +11,10 @@ def setup
     end
 
     def test_deprecation_output
-      output = ''
+      output = []
       SSHKit.config.deprecation_output = output
       SSHKit.config.deprecation_logger.log('Test')
-      assert_equal "[Deprecated] Test\n", output.lines.first
+      assert_equal "[Deprecated] Test\n", output.join.lines.first
     end
 
     def test_default_deprecation_output