从 NameValuePairs 列表中创建 UrlEncodedFormEntity 会引发 NullPointerException

我正在创建一个单元测试来尝试我刚刚创建的servlet。

@Test
public void test() throws ParseException, IOException {

  HttpClient client = new DefaultHttpClient();
  HttpPost post = new HttpPost("http://localhost:8080/WebService/MakeBaby");

  List<NameValuePair> nameValuePairs = new ArrayList<NameValuePair>();

  nameValuePairs.add(new BasicNameValuePair("father_name", "Foo"));
  nameValuePairs.add(new BasicNameValuePair("mother_name", "Bar"));

  post.setEntity(new UrlEncodedFormEntity(nameValuePairs));
  HttpResponse response = null;

  try {
    response = client.execute(post);
  } catch (ClientProtocolException e) {
    e.printStackTrace();
  } catch (IOException e) {
    e.printStackTrace();
  }

  String stringifiedResponse = EntityUtils.toString(response.getEntity());

  System.out.println(stringifiedResponse);

  assertNotNull(stringifiedResponse);
}

以下行生成 NullPointerException：

post.setEntity(new UrlEncodedFormEntity(nameValuePairs));

我错过了什么吗？