C initialize array within structure

I want to have an variable-length array contained within a structure, but am having trouble initializing it correctly.

struct Grid {
  int rows;
  int cols;
  int grid[];
}

int main() {
  struct Grid testgrid = {1, 3, {4, 5, 6}};
}

Everything I try gives me an 'error: non-static initialization of a flexible array member' error.