Can't JSON to javaBean

When Json to javaBean, I found an error. It's a NullPointerException. this is Json:

{
"errno": 0,
"msg": "success",
"data": {
    "direction": 0,
    "words_result_num": 19,
    "words_result": [
        {
            "words": "车辆管理产品资料库",
            "probability": {
                "variance": 0.006179,
                "average": 0.95664,
                "min": 0.746422
            }
        },
        {
            "words": "盛简体中文",
            "probability": {
                "variance": 0.052477,
                "average": 0.84753,
                "min": 0.407953
            }
        },
        {
            "words": "户姓名段红车辆:BW53 5 i ( wbasp2101cDR76734}内莎利号,BMEW43999",
            "probability": {
                "variance": 0.030252,
                "average": 0.856571,
                "min": 0.503605
            }
        },
        {
            "words": "保险公司",
            "probability": {
                "variance": 0.045034,
                "average": 0.847951,
                "min": 0.48522
            }
        }
    ]
}

}

This is my Json to javaBean method：

    public String OCRbyJSON(){
    OCRResultVo ObjResult = null ;
    if(StringUtils.isNotBlank(strJson)){
        try {
            JSONObject  jsStr = JSONObject.fromObject(strJson);
            jsStr.remove("unknown");

            ObjResult  = (OCRResultVo) JSONObject.toBean(jsStr,OCRResultVo.class);
            System.out.println("");
        } catch (Exception e) {
            e.printStackTrace();
        }
    }
    return NONE;
}

OCRResultVo Class:When I paste it, I omit the methods of Getting () and Setting ().And JSONObject is net.sf.json.JSONObject;

public class OCRResultVo {
private String errno;
private String msg;
private JSONObject data;

}

I am most confused is that if I remove JSON inside the "words_result": the content, there is no problem. So I do not know where it was wrong, is wrong with the type, or with the wrong way ........ If you need other information, please tell me. thanks everybody.