Question about error (Range with must be constant expression)

SystemVerilog
6

In reply to Chisiya:
If I give constant value, it works.
But assigning constant value to lsb_index before randomization call generates compile error.

bit [15:0] a, size_a, c;
int b;
int lsb_index;

lsb_index = 19;
std:: randomize(a) with {a >= b;
a[lsb_index : 0] == 0;
(a + size_a) <= c;};

I have compilation error " Range width must be constant expression " for line that is highlighted.

Any idea why I have that error and how I can resolve it?