Issues with VLA and Keil

Hi guys, for some reason even though I selected C99 mode in my target configuration, Keil insist on giving me "error: #70: incomplete type is not allowed". Since incomplete types are defined in C99, I would have guessed selecting it would prevent this error. Anybody know how to solve this issue, or is it just not allowed in Keil.

My struct looks something like this:
typedef struct foo_s
{ uint8_t arr[];
} foo_t;