Body Expectation with XPATH not working on mockserver

Question

I am using MockServer since a short time and trying to create an expectation on the Request-Body. My Body is an XML and I am using XPath to match the Request, but I can not make it work. The Expectation I defined:

[
  {
    "httpRequest": {
      "method": "POST",
      "headers": { "Content-Type": [ ".*/xml" ] },
      "path": "/apath",
      "body":{
            "type" : "XPATH",
            "xpath" : "bookstore"
        }
    },
    "httpResponse": {
            "statusCode": 200,
            "headers": [
                { "name": "Content-Type", "values": [ "text/xml" ] }
            ],
            "body": {
                "type": "XML",
                "xml": "<RS state=\"HAPPY DAYS!\"/>"
            }
    }
  }
]

I tried several combinations I have been sending the simple XML from www.w3schools.com

<bookstore>
<book>
  <title lang="en">Harry Potter</title>
    <price>29.99</price>
    </book>
    <book>
      <title lang="en">Learning XML</title><price>39.95</price>
</book>
</bookstore>

I tried matching on "xpath" : "bookstore", on "xpath" : "/bookstore" and as in https://www.mock-server.com/mock_server/creating_expectations.html#button_match_request_by_binary_body "xpath" : "/bookstore/book[price>30]/price" I always get the error "XPATH did not match".

Can anyone help?