Did you try nil instead of null?

I did and don't think it works because server returns unknown error and I'm not sure how to see what was sent, replacing map by cheshire output, works.

A quick echo server can show you what's being sent, but I think maybe a question would be what are you expecting to be sent? The http-kit docs recommend using 'your favorite encoder' to encode bodies/params. If the expectation is that http-kit will see a map and encode it to json, I think that might be where you're running into trouble. tl;dr if you want to send a json string, then using cheshire or data.json to create that json string would be the way to go afaik