Struct in Struct initialization

Question

Can somebody explain why pp1 compiles, but pp2 does not?

I suppose it has something to do with the union and that the compiler will try to fit everything inbetween { } into buffer8 instead of DATA?

#include <iostream>
using namespace std;


union PassFilterValues
{
    struct DATA 
    {
        int32_t a;
        int32_t b;
        int32_t c;
        int32_t k;
    } data;
    uint8_t buffer8[sizeof(DATA)];  
};

union PassFilterDynamic
{
    struct DATA
    {
        PassFilterValues lowFilter;
        PassFilterValues highFilter;
    } data; 
    uint8_t buffer8[sizeof(DATA)];
};

#define PASSFILTER_OFF                  {0,1,2,3}

int main() {
    PassFilterDynamic pp1 = {((PassFilterValues)PASSFILTER_OFF), ((PassFilterValues)PASSFILTER_OFF)};
    PassFilterDynamic pp2 = {PASSFILTER_OFF, PASSFILTER_OFF};
    return 0;
}

feel free to try here: http://ideone.com/XYhfIi