How to initialize a union?

If it's a struct then it can be done

*p = {var1, var2..};

But seems this doesn't work with union:

union Ptrlist
{
        Ptrlist *next;
            State *s;
};

Ptrlist *l;
l = allocate_space();
*l = {NULL};

Only to get:

expected expression before ‘{’ token