diff --git a/lib/http/2/huffman.rb b/lib/http/2/huffman.rb
index bc20dc7..5580813 100644
--- a/lib/http/2/huffman.rb
+++ b/lib/http/2/huffman.rb
@@ -30,7 +30,7 @@ def encode(str)
       # @return [String] binary string
       # @raise [CompressionError] when Huffman coded string is malformed
       def decode(buf)
-        emit = ''
+        emit = String.new
         state = 0 # start state
 
         mask = (1 << BITS_AT_ONCE) - 1
diff --git a/spec/framer_spec.rb b/spec/framer_spec.rb
index 0f73712..d3735f7 100644
--- a/spec/framer_spec.rb
+++ b/spec/framer_spec.rb
@@ -1,3 +1,4 @@
+# frozen-string-literal: false
 require 'helper'
 
 RSpec.describe HTTP2::Framer do
diff --git a/spec/helper.rb b/spec/helper.rb
index 99b8b55..4d7eeb8 100644
--- a/spec/helper.rb
+++ b/spec/helper.rb
@@ -1,3 +1,4 @@
+# frozen-string-literal: false
 require './spec/support/deep_dup'
 
 RSpec.configure(&:disable_monkey_patching!)