File.join() is our friend for joining paths

README.md

@@ -52,7 +52,7 @@ Zip::File.open(zipfile_name, Zip::File::CREATE) do |zipfile|
     # Two arguments:
     # - The name of the file as it will appear in the archive
     # - The original file, including the path to find it
-    zipfile.add(filename, folder + '/' + filename)
+    zipfile.add(filename, File.join(folder, filename))
   end
   zipfile.get_output_stream("myFile") { |os| os.write "myFile contains just this" }
 end