Retrofit Error - java.lang.IllegalArgumentException

URL Link : http://www.factsplanet.info/cities.php Method Retrofit Error: java.lang.IllegalArgumentException

Main Class

public class MainActivity extends AppCompatActivity {
    Retrofit retrofit;
    Factory service;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

         retrofit = new Retrofit.Builder()
                .baseUrl("https://factsplanet.info/")
                .build();

       service = retrofit.create(Factory.class);

        Call<ThisIsPojo> call = service.getnames();

        call.enqueue(new Callback<ThisIsPojo>() {
            @Override
            public void onResponse(Response<ThisIsPojo> response, Retrofit retrofit) {
                Toast.makeText(MainActivity.this, response.body().getData().get(1).getCountry(), Toast.LENGTH_SHORT).show();
            }

            @Override
            public void onFailure(Throwable t) {

            }
        });
    }
}

Interface Class:

public interface Factory {

@GET ("http://www.factsplanet.info/cities.php")
    Call<ThisIsPojo>  getnames();
}

POJO CLASS:

public class ThisIsPojo {

    private List<Datum> data = null;
    private Map<String, Object> additionalProperties = new HashMap<String, Object>();

    public List<Datum> getData() {
        return data;
    }

    public void setData(List<Datum> data) {
        this.data = data;
    }

    public Map<String, Object> getAdditionalProperties() {
        return this.additionalProperties;
    }

    public void setAdditionalProperty(String name, Object value) {
        this.additionalProperties.put(name, value);
    }

}

Error Says: java.lang.RuntimeException: Unable to start activity

java.lang.IllegalArgumentException: Unable to create converter for class app.com.alphaapps.android.suntest.pojo.ThisIsPojo for method Factory.getnames

Modify it like this:

retrofit = new Retrofit.Builder().baseUrl("https://factsplanet.info/").addConverterFactory(GsonConverterFactory.create()).build();