匿名联合体结构体在VC6.0环境不支持？

定义匿名联合体和结构体在VC6.0环境怎么总出错？

union
{
  unsigned char P1;
  struct
  {
    unsigned char P10 :1;
    unsigned char P11 :1;
    unsigned char P12 :1;
    unsigned char P13 :1;
    unsigned char P14 :1;
    unsigned char P15 :1;
    unsigned char P16 :1;
    unsigned char P17 :1;
  };
};

void main()
{
  P1 = 0xFF;
  P13 = 0;
}

上面这段程序在VC6.0里怎么总出错？是不支持吗？