Build error string

main.c

#include <stdio.h>

struct my_string_t {
    uint8_t len;
    // char string[];
    // const char string[];
    char string[128];
    // const char * const string;
};

#define STRING(_string) {.len=sizeof(_string), .string=(_string)}

int main(void)
{
    struct my_string_t my_string_list[] = {STRING("Hello")};

    fprintf(stderr, "%s\n", my_string_list[0].string);

    return 0;
}

When compiled with char string[] following build error:

$ gcc main.c
main.c: In function ‘main’:
main.c:11:56: error: non-static initialization of a flexible array member
 #define STRING(_string) {.len=sizeof(_string), .string=(_string)}
                                                        ^
main.c:15:44: note: in expansion of macro ‘STRING’
     struct my_string_t my_string_list[] = {STRING("Hello")};
                                            ^
main.c:11:56: note: (near initialization for ‘my_string_list[0]’)
 #define STRING(_string) {.len=sizeof(_string), .string=(_string)}
                                                        ^
main.c:15:44: note: in expansion of macro ‘STRING’
     struct my_string_t my_string_list[] = {STRING("Hello")};