diff --git a/.travis.yml b/.travis.yml
index 21a4c64f..b903c3b3 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -5,6 +5,7 @@ rvm:
   - 2.4
   - 2.5
   - 2.6
+  - 2.7
   - ruby-head
 matrix:
   fast_finish: true
diff --git a/lib/zip/crypto/decrypted_io.rb b/lib/zip/crypto/decrypted_io.rb
index ec9cab8b..1dab17c0 100644
--- a/lib/zip/crypto/decrypted_io.rb
+++ b/lib/zip/crypto/decrypted_io.rb
@@ -7,7 +7,7 @@ def initialize(io, decrypter)
       @decrypter = decrypter
     end
 
-    def read(length = nil, outbuf = '')
+    def read(length = nil, outbuf = +'')
       return ((length.nil? || length.zero?) ? "" : nil) if eof
 
       while length.nil? || (buffer.bytesize < length)
@@ -25,7 +25,7 @@ def eof
     end
 
     def buffer
-      @buffer ||= ''.dup
+      @buffer ||= +''
     end
 
     def input_finished?
diff --git a/lib/zip/entry.rb b/lib/zip/entry.rb
index f1963d8d..a24fb791 100644
--- a/lib/zip/entry.rb
+++ b/lib/zip/entry.rb
@@ -615,7 +615,7 @@ def create_file(dest_path, _continue_on_exists_proc = proc { Zip.continue_on_exi
         get_input_stream do |is|
           bytes_written = 0
           warned = false
-          buf = ''.dup
+          buf = +''
           while (buf = is.sysread(::Zip::Decompressor::CHUNK_SIZE, buf))
             os << buf
             bytes_written += buf.bytesize
diff --git a/lib/zip/extra_field.rb b/lib/zip/extra_field.rb
index 72c36764..0dcf0d5e 100644
--- a/lib/zip/extra_field.rb
+++ b/lib/zip/extra_field.rb
@@ -26,7 +26,7 @@ def extra_field_type_unknown(binstr, len, i)
     end
 
     def create_unknown_item
-      s = ''.dup
+      s = +''
       class << s
         alias_method :to_c_dir_bin, :to_s
         alias_method :to_local_bin, :to_s
diff --git a/lib/zip/inflater.rb b/lib/zip/inflater.rb
index bb735d11..c18f245c 100644
--- a/lib/zip/inflater.rb
+++ b/lib/zip/inflater.rb
@@ -3,7 +3,7 @@ class Inflater < Decompressor #:nodoc:all
     def initialize(*args)
       super
 
-      @buffer = ''.dup
+      @buffer = +''
       @zlib_inflater = ::Zlib::Inflate.new(-Zlib::MAX_WBITS)
     end