Nullpointer in ParserRegistry.getCharset()

private static String URL = 'https://sandbox.itunes.apple.com/verifyReceipt'

  // TODO: impl me
  public boolean isValid(String receipt, boolean sandbox = false) {
    //return true

    
    HTTPBuilder http = new HTTPBuilder(URL)

    boolean result = false


    http.request(POST, JSON) { req ->
      //body = ['receipt-data': receipt]
      body = ['receipt-data': 'hallo']

      response.success = { resp, json ->
        println 's'
        //log.debug('success')
        result = true
      }
      response.failure = { resp, json ->
        println 'f'
        //log.debug('failure')
        result = false
      }
    }

    result
  }

	/**
	 * Helper method to get the charset from the response.  This should be done 
	 * when manually parsing any text response to ensure it is decoded using the
	 * correct charset. For instance:<pre>
	 * Reader reader = new InputStreamReader( resp.getEntity().getContent(), 
	 *   ParserRegistry.getCharset( resp ) );</pre>
	 * @param resp
	 */
	public static String getCharset( HttpResponse resp ) {
        /* old code yields NP if resp.getEntity().getContentType() is null         
		NameValuePair charset = resp.getEntity().getContentType().getElements()[0].getParameterByName("charset");
		return ( charset == null || charset.getValue().trim().equals("") ) ?
			Charset.defaultCharset().name() : charset.getValue();
	    */

        // FIX: NP
        NameValuePair charset = null;
        Header header = resp.getEntity().getContentType();
        if (header != null) {
            HeaderElement[] headerElements = header.getElements();
            if (headerElements != null && headerElements.length > 0) {
                HeaderElement headerElement = headerElements[0];
                charset = headerElement.getParameterByName("charset");
            }
        }
        return ( charset == null || charset.getValue().trim().equals("") ) ?
			Charset.defaultCharset().name() : charset.getValue();
	}