[Rails] RubyZip zip file creation works on localhost but not heroku

I have rubyzip 0.9.9 and am creating a zip file like so:

Zip::ZipFile.open(zip_path, Zip::ZipFile::CREATE) do |zipfile|

    #code goes here

end

When I give it a zip path like this:  zip_path = File.join(Rails.root, "/tmp/zips/#{zip.id}.zip")  , it works fine locally, but not on heroku.  On heroku I get an error like: failed with Errno::ENOENT: No such file or directory - /tmp/zips/31.zip20140115-1059-1j1bx6

I looks like it's appending a timestamp onto the filepath for some reason.   I switched to using zip_path = File.expand_path('/tmp/zips/#{zip.id}.zip"), and I get that same "No such file or directory" error on localhost also.

What's going on here?  Something with the RubyZip gem?

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