diff --git a/lib/sprockets.rb b/lib/sprockets.rb
index 5f0b0c7b2..42d25a5f1 100644
--- a/lib/sprockets.rb
+++ b/lib/sprockets.rb
@@ -105,7 +105,7 @@ module Sprockets
   register_bundle_processor 'application/javascript', Bundle
   register_bundle_processor 'text/css', Bundle
 
-  register_bundle_metadata_reducer '*/*', :data, proc { "" }, :concat
+  register_bundle_metadata_reducer '*/*', :data, proc { "" }, :+
   register_bundle_metadata_reducer 'application/javascript', :data, proc { "" }, Utils.method(:concat_javascript_sources)
   register_bundle_metadata_reducer '*/*', :links, :+
 
diff --git a/lib/sprockets/directive_processor.rb b/lib/sprockets/directive_processor.rb
index f4680c4bb..fd0f16dce 100644
--- a/lib/sprockets/directive_processor.rb
+++ b/lib/sprockets/directive_processor.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: false
+
 require 'set'
 require 'shellwords'
 
diff --git a/lib/sprockets/server.rb b/lib/sprockets/server.rb
index 2ad2c9abd..15c01ba57 100644
--- a/lib/sprockets/server.rb
+++ b/lib/sprockets/server.rb
@@ -251,11 +251,11 @@ def cache_headers(env, etag)
         # If the request url contains a fingerprint, set a long
         # expires on the response
         if path_fingerprint(env["PATH_INFO"])
-          headers["Cache-Control"] << ", max-age=31536000"
+          headers["Cache-Control"] += ", max-age=31536000"
 
         # Otherwise set `must-revalidate` since the asset could be modified.
         else
-          headers["Cache-Control"] << ", must-revalidate"
+          headers["Cache-Control"] += ", must-revalidate"
           headers["Vary"] = "Accept-Encoding"
         end
 
diff --git a/lib/sprockets/utils.rb b/lib/sprockets/utils.rb
index 11ff9443e..114723cea 100644
--- a/lib/sprockets/utils.rb
+++ b/lib/sprockets/utils.rb
@@ -102,6 +102,7 @@ def string_end_with_semicolon?(str)
     #
     # Returns buf String.
     def concat_javascript_sources(buf, source)
+      buf = +buf
       if source.bytesize > 0
         buf << source
 
diff --git a/test/test_asset.rb b/test/test_asset.rb
index ae21c242a..52fc51dc0 100644
--- a/test/test_asset.rb
+++ b/test/test_asset.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: false
+
 require "sprockets_test"
 
 module AssetTests
diff --git a/test/test_digest_utils.rb b/test/test_digest_utils.rb
index 5d6d2184a..8f0b1e42b 100644
--- a/test/test_digest_utils.rb
+++ b/test/test_digest_utils.rb
@@ -35,7 +35,7 @@ def test_digest
     assert_equal "62427aa539a0b78e90fd710dc0e15f2960771ba44214b5d41d4a93a8b2940a38", hexdigest({"foo" => "baz"})
     assert_equal "b6054efd9929004bdd0a1c09eb2d12961325396da749143def3e9a4050aa703e", hexdigest([[:foo, 1]])
     assert_equal "79a19ffe41ecebd5dc35e95363e0b4aa79b139a22bc650384df57eb09842f099", hexdigest([{:foo => 1}])
-    assert_equal "94ee40cca7c2c6d2a134033d2f5a31c488cad5d3dcc61a3dbb5e2a858635874b", hexdigest("foo".force_encoding('UTF-8').encoding)
+    assert_equal "94ee40cca7c2c6d2a134033d2f5a31c488cad5d3dcc61a3dbb5e2a858635874b", hexdigest(Encoding::UTF_8)
 
     assert_raises(TypeError) do
       digest(Object.new)
diff --git a/test/test_encoding.rb b/test/test_encoding.rb
index 5e9e940b2..1115771ab 100644
--- a/test/test_encoding.rb
+++ b/test/test_encoding.rb
@@ -1,4 +1,5 @@
 # encoding: utf-8
+# frozen_string_literal: false
 require "sprockets_test"
 
 class AssetEncodingTest < Sprockets::TestCase
diff --git a/test/test_sass.rb b/test/test_sass.rb
index 6c6a38a88..500fa91a5 100644
--- a/test/test_sass.rb
+++ b/test/test_sass.rb
@@ -1,5 +1,7 @@
 require 'sprockets_test'
 
+return if "test".frozen? # The sass gem isn't frozen string literal compatible
+
 silence_warnings do
   require 'sass'
 end