https://itmanual.net/%ec%97%91%ec%85%80-n-a-ref-name-null-%eb%8d%b0%ec%9d%b4%ed%84%b0-%ec%b2%98%eb%a6%ac-%ea%b2%80%ec%83%89-%ea%b4%80%eb%a0%a8-%ec%98%a4%eb%a5%98/