error: can't convert Symbol into Integer

[rattrayalex]

When I run the following command:
http --form -f POST http://c.docverter.com/convert from=pdf to=html 'input_files[]="$(cat my_file.pdf)"' --output out.html
I get the following error:
{"error":"can't convert Symbol into Integer"}

Note that I am attempting to use cat here because of #23

[rattrayalex]

(the single quotes are because I'm using ZSH, but I got the same error using bash & curl)