Pointer to array in a structure?

hello,

i have a simple little structure like this

unsigned char index;

struct tag {
unsigned char a[20];
unsigned char b[20];
unsigned char c
};

then i declare a pointer like this

struct tag TAG_PTR;

it seems that TAG_PTR->c; is valid and generates no errors

however TAG_PTR->a[index]; generated BAD OPPERAND TYPE error

and TAG_PTR->a[10];(or any other constant value)
and TAG_PTR->a; generate no errors.

obviously using the index variable is the problem, but im not sure how to work around it. Any help would be greatly appreciated.