Array initialization inside struct

Hello,
I try compile this code (.c) with --c99 option

struct sx { char c; char a[];
};
static struct sx xx = { 1, {[5] = 0} };

and have error
error: #146: too many initializer values

How to initialize array inside struct ?

ARM C/C++ Compiler, 4.1 [Build 791] [Standard]

Thanks