Lexical error in multipart request

I am trying to perform a multipart PUT request to CouchDB, but am receiving the following error.

[Sun, 26 Mar 2017 16:21:04 GMT] [debug] [<0.6514.2>] 'PUT' /couch-recliner-test/fake-id {1,1} from "127.0.0.1"
Headers: [{'Accept',"application/json"},
          {'Connection',"close"},
          {'Content-Length',"665"},
          {'Content-Type',"multipart/form-data; boundary=--------------------NODENEEDLEHTTPCLIENT"},
          {'Host',"localhost:5984"},
          {'User-Agent',"Needle/1.5.0 (Node.js v7.7.3; linux x64)"}]
[Sun, 26 Mar 2017 16:21:04 GMT] [debug] [<0.6514.2>] OAuth Params: []
[Sun, 26 Mar 2017 16:21:04 GMT] [error] [<0.6514.2>] attempted upload of invalid JSON (set log_level to debug to log it)
[Sun, 26 Mar 2017 16:21:04 GMT] [debug] [<0.6514.2>] Invalid JSON: {{error,
                                     {1,
                                      "lexical error: malformed number, a digit is required after the minus sign.\n"}},
                                    <<"----------------------NODENEEDLEHTTPCLIENT\r\nContent-Disposition: form-data; name=\"document_body\"\r\nContent-Type: application/json\r\n\r\n{\"test\":\"data\",\"hello\":22,\"deep\":{\"more\":\"hii\",\"array\":[22,33,145]},\"array\":[\"some\",\"text\",\"here\"],\"description\":\"A longer bit of text with perhaps a line break in it.\\nRight here.\",\"bool\":true,\"_attachments\":{\"fake-attachment.txt\":{\"follows\":true,\"content_type\":\"text/html\",\"length\":46}}}\r\n----------------------NODENEEDLEHTTPCLIENT\r\nContent-Disposition: form-data; name=\"file_0\"; filename=\"fake-attachment.txt\"\r\nContent-Type: text/html\r\n\r\nThis is an example attachment containing text.\r\n----------------------NODENEEDLEHTTPCLIENT--">>}
[Sun, 26 Mar 2017 16:21:04 GMT] [info] [<0.6514.2>] 127.0.0.1 - - PUT /couch-recliner-test/fake-id 400
[Sun, 26 Mar 2017 16:21:04 GMT] [debug] [<0.6514.2>] httpd 400 error response:
 {"error":"bad_request","reason":"invalid_json"}

The request looks approximately like this.

const mydata = {
    document_body: {
        content_type: 'application/json',
        value: '{"test":"data","hello":22,"deep":{"more":"hii","array":[22,33,145]},"array":["some","text","here"],"description":"A longer bit of text with perhaps a line break in it.\\nRight here.","bool":true,"_attachments":{"fake-attachment.txt":{"follows":true,"content_type":"text/html","length":46}}}'
    },
    file_0: {
        content_type: 'text/html',
        filename: 'fake-attachment.txt',
        buffer: mybuffer
    }
};

needle.request('PUT', url, mydata, { multipart: true, json: true }, callback);

The key part of the error seems to be.

"lexical error: malformed number, a digit is required after the minus sign.\n"

But I am unsure what it means.

Start at the needle.request('PUT', url, mydata, { multipart: true, json: true }, callback) entry point and reproduce the multipart request against CouchDB using the example payload. Trace how the document_body value and multipart boundaries are assembled; done means the request no longer produces CouchDB's invalid_json lexical error.