Add body to luasocket POST request with generic form?

Question

From https://w3.impa.br/~diego/software/luasocket/http.html, there are two ways to make a request, simple and generic. I have gotten the body to work with the simple method. However, when I add an LTN12 source to the generic method, an empty body is sent to the server.

http.request(url [, body])

http.request{
  url = string,
  [sink = LTN12 sink,]
  [method = string,]
  [headers = header-table,]
  [source = LTN12 source],
  [step = LTN12 pump step,]
  [proxy = string,]
  [redirect = boolean,]
  [create = function]
}

This works:

http.request("http://localhost:56218/sendState", "at=" .. AT)

This doesn't:

    local reqbody = "hi"
    local respbody = {} 
    local  body, code, headers, status = http.request {
      url = "http://localhost:56218/sendState",
      source = ltn12.source.string(reqBody),
      headers = {
        ["content-length"] = string.len(reqbody)
      }
      sink = ltn12.sink.table(respbody)
  }

When I try to read the body of the above line of code in my server, it is empty. What am I doing wrong?