Parse Error: Expected HTTP/

On mockttp 0.20.0, 0.20.1, and 0.20.2 I get a weird http parse error. I think it is something to do with response encoding.

Node version is 12.18.0

tested the request with request-promise, axios, and fetch with the same results each time

{
  "config": {
    "url": "https://localhost:1234/oauth/access_token.php",
    "method": "post",
    "data": {
      "_overheadLength": 432,
      "_valueLength": 150,
      "_valuesToMeasure": [],
      "writable": false,
      "readable": true,
      "dataSize": 0,
      "maxDataSize": 2097152,
      "pauseStreams": true,
      "_released": true,
      "_streams": [],
      "_currentStream": null,
      "_insideLoop": false,
      "_pendingNext": false,
      "_boundary": "--------------------------229731192039541199026626",
      "_events": {},
      "_eventsCount": 1
    },
    "headers": {
      "Accept": "application/json, text/plain, */*",
      "Content-Type": "multipart/form-data; boundary=--------------------------229731192039541199026626",
      "User-Agent": "axios/0.19.2"
    },
    "transformRequest": [
      null
    ],
    "transformResponse": [
      null
    ],
    "timeout": 0,
    "xsrfCookieName": "XSRF-TOKEN",
    "xsrfHeaderName": "X-XSRF-TOKEN",
    "maxContentLength": -1
  },
  "code": "HPE_INVALID_CONSTANT"
}