diff --git a/lib/zip/crypto/decrypted_io.rb b/lib/zip/crypto/decrypted_io.rb
index ec9cab8..362e4a4 100644
--- a/lib/zip/crypto/decrypted_io.rb
+++ b/lib/zip/crypto/decrypted_io.rb
@@ -7,7 +7,7 @@ module Zip
       @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)