PDFkit raise "command failed: #{invoke}" if result.to_s.strip.empty?

Question

I'm trying to use PDFkit. I get

command failed: "/Users/burtondav/sites/ndeavor/bin/wkhtmltopdf-amd64" "--page-size" "Letter" "--margin-top" "0.75in" "--margin-right" "0.75in" "--margin-bottom" "0.75in" "--margin-left" "0.75in" "--encoding" "UTF-8" "--print-media-type" "--quiet" "-" "-"

It errors on this code in (gem) pdfkit-0.5.3/lib/pdfkit/pdfkit.rb

73     raise "command failed: #{invoke}" if result.to_s.strip.empty?

I have wkhtmltopdf-amd64 in /bin/

In pdfkit.rb, I have:

 config.wkhtmltopdf = Rails.root.join('bin', 'wkhtmltopdf-amd64').to_s

Thanks for the help!