c - Why is strlen working fine without ''?

As title, I have some question using char* in c. For example, if I write this

char *a = calloc(5, 5);
a[0] = '1';
a[1] = '1';
a[2] = '1';
a[3] = '1';
a[4] = '1';

printf("a = %s, length = %d", a, strlen(a));

and the output is

a = 11111, length = 5

Why is strlen working fine without '\0'? Can someone help me understand?

Answer

calloc(5, 5) allocates and zeroes 25 bytes. You assign the first five of these, but the sixth is still '\0'.