Retrofit返回null，需要字符串

我是一个完整的Android初学者。以下是各种组件。总的来说，我正在尝试在给定用户名的情况下获取列的值（作为参数输入）。

Call<DefaultResponse> call2 = RetrofitClient.getInstance().getApi().getInformation("fullname","billy@gmail.com");

    call2.enqueue(new Callback<DefaultResponse>() {
        @Override
        public void onResponse(Call<DefaultResponse> call2, Response<DefaultResponse> response) {

            String informationReturned = response.body().getMsg();
            textView.append("\n"+informationReturned);
        }


        @Override
        public void onFailure(Call<DefaultResponse> call2, Throwable t) {

        }


    });

DefaultResponse类：

import com.google.gson.annotations.SerializedName;

public class DefaultResponse {

    @SerializedName("error")
    private boolean err;

    @SerializedName("message")
    private String msg;

    public DefaultResponse(boolean err, String msg) {
        this.err = err;
        this.msg = msg;
    }

    public boolean isErr() {
        return err;
    }

    public String getMsg() {
        return msg;
    }
}

获得电话

@GET("getInformation/{field}/{username}")
Call<DefaultResponse> getInformation(@Path("field") String field, @Path("username") String username);

The PHP itself works great, as confirmed by Postman when I entered GET command http://localhost/MyApi/public/getInformation/fullname/billy@gmail.com