diff --git a/lib/combine_pdf/basic_writer.rb b/lib/combine_pdf/basic_writer.rb
index 30661e4..14a6ab7 100644
--- a/lib/combine_pdf/basic_writer.rb
+++ b/lib/combine_pdf/basic_writer.rb
@@ -36,7 +36,7 @@ class PDFWriter < Hash
     # mediabox:: the PDF page size in PDF points. defaults to [0, 0, 612.0, 792.0] (US Letter)
     def initialize(mediabox = [0, 0, 612.0, 792.0])
       # indirect_reference_id, :indirect_generation_number
-      @contents = ''
+      @contents = String.new
       @base_font_name = 'Writer' + SecureRandom.hex(7) + 'PDF'
       self[:Type] = :Page
       self[:indirect_reference_id] = 0