diff --git a/Gemfile b/Gemfile
index b733b8a..2168bc7 100644
--- a/Gemfile
+++ b/Gemfile
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 # Copyright 2017 Holger Just
 #
 # This software may be modified and distributed under the terms
diff --git a/lib/rackstash.rb b/lib/rackstash.rb
index 6710488..60dae1e 100644
--- a/lib/rackstash.rb
+++ b/lib/rackstash.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 # Copyright 2017 Holger Just
 #
 # This software may be modified and distributed under the terms
diff --git a/lib/rackstash/adapters.rb b/lib/rackstash/adapters.rb
index f8d5730..54ac7fd 100644
--- a/lib/rackstash/adapters.rb
+++ b/lib/rackstash/adapters.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 # Copyright 2017 Holger Just
 #
 # This software may be modified and distributed under the terms
diff --git a/lib/rackstash/adapters/adapter.rb b/lib/rackstash/adapters/adapter.rb
index ff03c25..066e914 100644
--- a/lib/rackstash/adapters/adapter.rb
+++ b/lib/rackstash/adapters/adapter.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 # Copyright 2017 Holger Just
 #
 # This software may be modified and distributed under the terms
@@ -97,10 +99,11 @@ module Rackstash
       # that ends in a newline character by mutating the object is required.
       #
       # @param line [#to_s] a log line
-      # @return [String] a string with a trailing newline character (`"\n"`)
+      # @return [String] `line` with a trailing newline character (`"\n"`)
+      #   appended if necessary
       def normalize_line(line)
         line = line.to_s
-        line << "\n".freeze unless line.end_with?("\n".freeze)
+        line = "#{line}\n" unless line.end_with?("\n".freeze)
         line
       end
     end
diff --git a/lib/rackstash/adapters/callable.rb b/lib/rackstash/adapters/callable.rb
index 9a129ec..0f95d36 100644
--- a/lib/rackstash/adapters/callable.rb
+++ b/lib/rackstash/adapters/callable.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 # Copyright 2017 Holger Just
 #
 # This software may be modified and distributed under the terms
diff --git a/lib/rackstash/adapters/file.rb b/lib/rackstash/adapters/file.rb
index ef8bb53..eae480a 100644
--- a/lib/rackstash/adapters/file.rb
+++ b/lib/rackstash/adapters/file.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 # Copyright 2017 Holger Just
 #
 # This software may be modified and distributed under the terms
diff --git a/lib/rackstash/adapters/io.rb b/lib/rackstash/adapters/io.rb
index 61a202d..7fd32b9 100644
--- a/lib/rackstash/adapters/io.rb
+++ b/lib/rackstash/adapters/io.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 # Copyright 2017 Holger Just
 #
 # This software may be modified and distributed under the terms
diff --git a/lib/rackstash/buffer.rb b/lib/rackstash/buffer.rb
index 1e978fd..518ce7d 100644
--- a/lib/rackstash/buffer.rb
+++ b/lib/rackstash/buffer.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 # Copyright 2017 Holger Just
 #
 # This software may be modified and distributed under the terms
diff --git a/lib/rackstash/buffer_stack.rb b/lib/rackstash/buffer_stack.rb
index 46381f2..40eda59 100644
--- a/lib/rackstash/buffer_stack.rb
+++ b/lib/rackstash/buffer_stack.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 # Copyright 2017 Holger Just
 #
 # This software may be modified and distributed under the terms
diff --git a/lib/rackstash/encoders.rb b/lib/rackstash/encoders.rb
index 056824b..816528a 100644
--- a/lib/rackstash/encoders.rb
+++ b/lib/rackstash/encoders.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 # Copyright 2017 Holger Just
 #
 # This software may be modified and distributed under the terms
diff --git a/lib/rackstash/encoders/json.rb b/lib/rackstash/encoders/json.rb
index 25b475f..b6442c2 100644
--- a/lib/rackstash/encoders/json.rb
+++ b/lib/rackstash/encoders/json.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 # Copyright 2017 Holger Just
 #
 # This software may be modified and distributed under the terms
@@ -21,7 +23,7 @@ module Rackstash
       # @param event [Hash] a log event as produced by the {Flow}
       # @return [String] the event as a single-line JSON string
       def encode(event)
-        event[FIELD_MESSAGE].strip!
+        event[FIELD_MESSAGE] = event[FIELD_MESSAGE].strip
         ::JSON.dump(event)
       end
     end
diff --git a/lib/rackstash/encoders/message.rb b/lib/rackstash/encoders/message.rb
index f7615c7..5833dae 100644
--- a/lib/rackstash/encoders/message.rb
+++ b/lib/rackstash/encoders/message.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 # Copyright 2017 Holger Just
 #
 # This software may be modified and distributed under the terms
diff --git a/lib/rackstash/encoders/raw.rb b/lib/rackstash/encoders/raw.rb
index 82fd267..8608901 100644
--- a/lib/rackstash/encoders/raw.rb
+++ b/lib/rackstash/encoders/raw.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 # Copyright 2017 Holger Just
 #
 # This software may be modified and distributed under the terms
diff --git a/lib/rackstash/fields.rb b/lib/rackstash/fields.rb
index 8c9d35a..6bfb7a6 100644
--- a/lib/rackstash/fields.rb
+++ b/lib/rackstash/fields.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 # Copyright 2017 Holger Just
 #
 # This software may be modified and distributed under the terms
diff --git a/lib/rackstash/fields/abstract_collection.rb b/lib/rackstash/fields/abstract_collection.rb
index cd4d9d8..7a8068e 100644
--- a/lib/rackstash/fields/abstract_collection.rb
+++ b/lib/rackstash/fields/abstract_collection.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 # Copyright 2017 Holger Just
 #
 # This software may be modified and distributed under the terms
@@ -156,7 +158,7 @@ module Rackstash
           return utf8_encode(value)
         when Exception
           exception = "#{value.message} (#{value.class})"
-          exception << "\n" << value.backtrace.join("\n") if value.backtrace
+          exception = [exception, *value.backtrace].join("\n") if value.backtrace
           return utf8_encode(exception)
         when ::Proc
           return normalize(value, scope: scope, wrap: wrap)
diff --git a/lib/rackstash/fields/array.rb b/lib/rackstash/fields/array.rb
index 3260904..d1955c5 100644
--- a/lib/rackstash/fields/array.rb
+++ b/lib/rackstash/fields/array.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 # Copyright 2017 Holger Just
 #
 # This software may be modified and distributed under the terms
diff --git a/lib/rackstash/fields/hash.rb b/lib/rackstash/fields/hash.rb
index 9b14060..de469e1 100644
--- a/lib/rackstash/fields/hash.rb
+++ b/lib/rackstash/fields/hash.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 # Copyright 2017 Holger Just
 #
 # This software may be modified and distributed under the terms
diff --git a/lib/rackstash/fields/tags.rb b/lib/rackstash/fields/tags.rb
index bd476e6..3b76623 100644
--- a/lib/rackstash/fields/tags.rb
+++ b/lib/rackstash/fields/tags.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 # Copyright 2017 Holger Just
 #
 # This software may be modified and distributed under the terms
diff --git a/lib/rackstash/flow.rb b/lib/rackstash/flow.rb
index 783a311..ca8281c 100644
--- a/lib/rackstash/flow.rb
+++ b/lib/rackstash/flow.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 # Copyright 2017 Holger Just
 #
 # This software may be modified and distributed under the terms
diff --git a/lib/rackstash/flows.rb b/lib/rackstash/flows.rb
index f3c95a7..5dc130e 100644
--- a/lib/rackstash/flows.rb
+++ b/lib/rackstash/flows.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 # Copyright 2017 Holger Just
 #
 # This software may be modified and distributed under the terms
diff --git a/lib/rackstash/formatter.rb b/lib/rackstash/formatter.rb
index b51378b..0fb8878 100644
--- a/lib/rackstash/formatter.rb
+++ b/lib/rackstash/formatter.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 # Copyright 2017 Holger Just
 #
 # This software may be modified and distributed under the terms
diff --git a/lib/rackstash/logger.rb b/lib/rackstash/logger.rb
index ab62675..6b6d424 100644
--- a/lib/rackstash/logger.rb
+++ b/lib/rackstash/logger.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 # Copyright 2017 Holger Just
 #
 # This software may be modified and distributed under the terms
diff --git a/lib/rackstash/message.rb b/lib/rackstash/message.rb
index 8059cf4..0c5de1e 100644
--- a/lib/rackstash/message.rb
+++ b/lib/rackstash/message.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 # Copyright 2017 Holger Just
 #
 # This software may be modified and distributed under the terms
diff --git a/lib/rackstash/sink.rb b/lib/rackstash/sink.rb
index 66ff9cc..2180a18 100644
--- a/lib/rackstash/sink.rb
+++ b/lib/rackstash/sink.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 # Copyright 2017 Holger Just
 #
 # This software may be modified and distributed under the terms
diff --git a/lib/rackstash/version.rb b/lib/rackstash/version.rb
index 66a9251..b4387e3 100644
--- a/lib/rackstash/version.rb
+++ b/lib/rackstash/version.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 # Copyright 2017 Holger Just
 #
 # This software may be modified and distributed under the terms
diff --git a/rackstash.gemspec b/rackstash.gemspec
index 91558ca..3fd1a69 100644
--- a/rackstash.gemspec
+++ b/rackstash.gemspec
@@ -1,4 +1,5 @@
-# coding: utf-8
+# frozen_string_literal: true
+
 # Copyright 2017 Holger Just
 #
 # This software may be modified and distributed under the terms
diff --git a/spec/rackstash/adapters/adapter_spec.rb b/spec/rackstash/adapters/adapter_spec.rb
index 62547db..710d6e3 100644
--- a/spec/rackstash/adapters/adapter_spec.rb
+++ b/spec/rackstash/adapters/adapter_spec.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 # Copyright 2017 Holger Just
 #
 # This software may be modified and distributed under the terms
diff --git a/spec/rackstash/adapters/callable_spec.rb b/spec/rackstash/adapters/callable_spec.rb
index d453be6..a2d0f89 100644
--- a/spec/rackstash/adapters/callable_spec.rb
+++ b/spec/rackstash/adapters/callable_spec.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 # Copyright 2017 Holger Just
 #
 # This software may be modified and distributed under the terms
diff --git a/spec/rackstash/adapters/file_spec.rb b/spec/rackstash/adapters/file_spec.rb
index feffdc5..0871c67 100644
--- a/spec/rackstash/adapters/file_spec.rb
+++ b/spec/rackstash/adapters/file_spec.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 # Copyright 2017 Holger Just
 #
 # This software may be modified and distributed under the terms
diff --git a/spec/rackstash/adapters/io_spec.rb b/spec/rackstash/adapters/io_spec.rb
index 6ac501d..ba663c7 100644
--- a/spec/rackstash/adapters/io_spec.rb
+++ b/spec/rackstash/adapters/io_spec.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 # Copyright 2017 Holger Just
 #
 # This software may be modified and distributed under the terms
diff --git a/spec/rackstash/adapters_spec.rb b/spec/rackstash/adapters_spec.rb
index 1594308..bab9cf4 100644
--- a/spec/rackstash/adapters_spec.rb
+++ b/spec/rackstash/adapters_spec.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 # Copyright 2017 Holger Just
 #
 # This software may be modified and distributed under the terms
diff --git a/spec/rackstash/buffer_spec.rb b/spec/rackstash/buffer_spec.rb
index 6994f23..ff7e2ff 100644
--- a/spec/rackstash/buffer_spec.rb
+++ b/spec/rackstash/buffer_spec.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 # Copyright 2017 Holger Just
 #
 # This software may be modified and distributed under the terms
diff --git a/spec/rackstash/buffer_stack_spec.rb b/spec/rackstash/buffer_stack_spec.rb
index d055fc6..59b544a 100644
--- a/spec/rackstash/buffer_stack_spec.rb
+++ b/spec/rackstash/buffer_stack_spec.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 # Copyright 2017 Holger Just
 #
 # This software may be modified and distributed under the terms
diff --git a/spec/rackstash/encoders/json_spec.rb b/spec/rackstash/encoders/json_spec.rb
index 67433e1..bc87943 100644
--- a/spec/rackstash/encoders/json_spec.rb
+++ b/spec/rackstash/encoders/json_spec.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 # Copyright 2017 Holger Just
 #
 # This software may be modified and distributed under the terms
diff --git a/spec/rackstash/encoders/message_spec.rb b/spec/rackstash/encoders/message_spec.rb
index d28a75f..c1a65b6 100644
--- a/spec/rackstash/encoders/message_spec.rb
+++ b/spec/rackstash/encoders/message_spec.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 # Copyright 2017 Holger Just
 #
 # This software may be modified and distributed under the terms
diff --git a/spec/rackstash/encoders/raw_spec.rb b/spec/rackstash/encoders/raw_spec.rb
index abdb788..6ded912 100644
--- a/spec/rackstash/encoders/raw_spec.rb
+++ b/spec/rackstash/encoders/raw_spec.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 # Copyright 2017 Holger Just
 #
 # This software may be modified and distributed under the terms
diff --git a/spec/rackstash/fields/abstract_collection_spec.rb b/spec/rackstash/fields/abstract_collection_spec.rb
index 1606202..c79c0f7 100644
--- a/spec/rackstash/fields/abstract_collection_spec.rb
+++ b/spec/rackstash/fields/abstract_collection_spec.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 # Copyright 2017 Holger Just
 #
 # This software may be modified and distributed under the terms
@@ -63,7 +65,7 @@ describe Rackstash::Fields::AbstractCollection do
 
   describe '#clone' do
     it 'clones the raw value' do
-      raw = 'hello'
+      raw = []
       collection.send(:raw=, raw)
       expect(collection.send(:raw)).to equal raw
 
@@ -71,14 +73,14 @@ describe Rackstash::Fields::AbstractCollection do
       cloned = collection.clone
 
       expect(cloned).not_to equal collection
-      expect(cloned.send(:raw)).to eql 'hello'
+      expect(cloned.send(:raw)).to eql raw
       expect(cloned.send(:raw)).not_to equal raw
     end
   end
 
   describe '#dup' do
     it 'dups the raw value' do
-      raw = 'hello'
+      raw = []
       collection.send(:raw=, raw)
       expect(collection.send(:raw)).to equal raw
 
@@ -86,7 +88,7 @@ describe Rackstash::Fields::AbstractCollection do
       duped = collection.dup
 
       expect(duped).not_to equal collection
-      expect(duped.send(:raw)).to eql 'hello'
+      expect(duped.send(:raw)).to eql raw
       expect(duped.send(:raw)).not_to equal raw
     end
   end
diff --git a/spec/rackstash/fields/array_spec.rb b/spec/rackstash/fields/array_spec.rb
index 41454ab..0833082 100644
--- a/spec/rackstash/fields/array_spec.rb
+++ b/spec/rackstash/fields/array_spec.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 # Copyright 2017 Holger Just
 #
 # This software may be modified and distributed under the terms
diff --git a/spec/rackstash/fields/hash_spec.rb b/spec/rackstash/fields/hash_spec.rb
index 4547008..c1660f4 100644
--- a/spec/rackstash/fields/hash_spec.rb
+++ b/spec/rackstash/fields/hash_spec.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 # Copyright 2017 Holger Just
 #
 # This software may be modified and distributed under the terms
diff --git a/spec/rackstash/fields/tags_spec.rb b/spec/rackstash/fields/tags_spec.rb
index c98c205..93e9d58 100644
--- a/spec/rackstash/fields/tags_spec.rb
+++ b/spec/rackstash/fields/tags_spec.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 # Copyright 2017 Holger Just
 #
 # This software may be modified and distributed under the terms
diff --git a/spec/rackstash/flows_spec.rb b/spec/rackstash/flows_spec.rb
index ad4fecf..297eba7 100644
--- a/spec/rackstash/flows_spec.rb
+++ b/spec/rackstash/flows_spec.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 # Copyright 2017 Holger Just
 #
 # This software may be modified and distributed under the terms
diff --git a/spec/rackstash/formatter_spec.rb b/spec/rackstash/formatter_spec.rb
index ea7e0bf..cc0a0ac 100644
--- a/spec/rackstash/formatter_spec.rb
+++ b/spec/rackstash/formatter_spec.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 # Copyright 2017 Holger Just
 #
 # This software may be modified and distributed under the terms
diff --git a/spec/rackstash/logger_spec.rb b/spec/rackstash/logger_spec.rb
index 2b9bcda..3b9153f 100644
--- a/spec/rackstash/logger_spec.rb
+++ b/spec/rackstash/logger_spec.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 # Copyright 2017 Holger Just
 #
 # This software may be modified and distributed under the terms
diff --git a/spec/rackstash/message_spec.rb b/spec/rackstash/message_spec.rb
index 03b9e2c..bed3712 100644
--- a/spec/rackstash/message_spec.rb
+++ b/spec/rackstash/message_spec.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 # Copyright 2017 Holger Just
 #
 # This software may be modified and distributed under the terms
@@ -108,8 +110,8 @@ describe Rackstash::Message do
   end
 
   describe '#progname' do
-    it 'dups the progname' do
-      progname = 'a message'
+    it 'dup-freezes a mutable progname' do
+      progname = String.new('a message')
       message = Rackstash::Message.new('', progname: progname)
 
       expect(message.progname).to eql progname
diff --git a/spec/rackstash/sink_spec.rb b/spec/rackstash/sink_spec.rb
index 1ea79c9..0666562 100644
--- a/spec/rackstash/sink_spec.rb
+++ b/spec/rackstash/sink_spec.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 # Copyright 2017 Holger Just
 #
 # This software may be modified and distributed under the terms
diff --git a/spec/rackstash/version_spec.rb b/spec/rackstash/version_spec.rb
index 2917640..1a64c4c 100644
--- a/spec/rackstash/version_spec.rb
+++ b/spec/rackstash/version_spec.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 # Copyright 2017 Holger Just
 #
 # This software may be modified and distributed under the terms
diff --git a/spec/rackstash_spec.rb b/spec/rackstash_spec.rb
index e02bce3..7da0266 100644
--- a/spec/rackstash_spec.rb
+++ b/spec/rackstash_spec.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 # Copyright 2017 Holger Just
 #
 # This software may be modified and distributed under the terms
diff --git a/spec/spec_helper.rb b/spec/spec_helper.rb
index 4e4c291..57e0df9 100644
--- a/spec/spec_helper.rb
+++ b/spec/spec_helper.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 # Copyright 2017 Holger Just
 #
 # This software may be modified and distributed under the terms