Mockaroo API call in Java

Question

I tried calling Mockaroo API in my JAVA Class as per in Mockaroo Doc,But it shows the

Server returned HTTP response code: 500 for URL

Hereby I have attached my code, Do I miss the Parameter/any Query Strings?

URL url = new URL("https://api.mockaroo.com/api/generate.json?key=abcd1234");

HttpURLConnection conn = (HttpURLConnection) url.openConnection();
conn.setDoOutput(true);
conn.setRequestMethod("POST");
conn.setRequestProperty("Content-Type", "application/json");

JSONObject yearsEmployed = new JSONObject();
yearsEmployed.put("name", "yearsEmployed");
yearsEmployed.put("type", "Number");
yearsEmployed.put("min", 1);
yearsEmployed.put("max", 30);
yearsEmployed.put("decimals", 0);

JSONObject department = new JSONObject();
department.put("name", "department");
department.put("type", "Custom List");
JSONArray values = new JSONArray();
values.put("R+D");
values.put("Marketing");
values.put("HR");
department.put("values", values);

JSONObject dob = new JSONObject();
dob.put("name", "dob");
dob.put("type", "Date");
dob.put("min", "1/1/1950");
dob.put("max", "1/1/2000");
dob.put("format", "%m/%d/%Y");

JSONArray fields = new JSONArray();
fields.put(yearsEmployed);
fields.put(department);
fields.put(dob);

OutputStream os = conn.getOutputStream();
os.write(fields.toString().getBytes());
os.flush();

JSONObject data = new JSONObject(IOUtils.toString(conn.getInputStream()));

System.out.println(data.getInt("yearsEmployed"));
System.out.println(data.getString("department"));
System.out.println(data.getString("dob"));

And here my error code,

Exception in thread "main" java.io.IOException: Server returned HTTP response code: 500 for URL: https://api.mockaroo.com/api/generate.json?key=abcd1234
    at java.base/sun.net.www.protocol.http.HttpURLConnection.getInputStream0(HttpURLConnection.java:1924)
    at java.base/sun.net.www.protocol.http.HttpURLConnection.getInputStream(HttpURLConnection.java:1520)
    at java.base/sun.net.www.protocol.https.HttpsURLConnectionImpl.getInputStream(HttpsURLConnectionImpl.java:250)
    at com.cs4221.dataGeneration.randomData.main(randomData.java:61)