diff --git a/lib/combine_pdf/pdf_public.rb b/lib/combine_pdf/pdf_public.rb
index 43c564b..b4c910e 100644
--- a/lib/combine_pdf/pdf_public.rb
+++ b/lib/combine_pdf/pdf_public.rb
@@ -207,7 +207,7 @@ def to_pdf(options = {})
       # xref_location = 0
       # out.each { |line| xref_location += line.bytesize + 1}
       out << "xref\n0 #{indirect_object_count}\n0000000000 65535 f "
-      xref.each { |offset| out << ("%010d 00000 n " % offset) }
+      xref.each { |offset| out << ("%010d 00000 n ".freeze % offset) }
       out << 'trailer'.freeze
       out << "<<\n/Root #{false || "#{catalog[:indirect_reference_id]} #{catalog[:indirect_generation_number]} R"}"
       out << "/Size #{indirect_object_count}"