Nested Varnish directors errors

test.vcl

vcl 4.0;

import directors;
backend alpha { .host = "192.168.0.101"; }
backend bravo { .host = "192.168.0.102"; }

sub vcl_init {
    new primary_director = directors.random();
    primary_director.add_backend(alpha, 1.0);

    new secondary_director = directors.random();
    secondary_director.add_backend(bravo, 1.0);

    new fallback_director = directors.fallback();
    fallback_director.add_backend(primary_director);
    fallback_director.add_backend(secondary_director);
}

varnish_4.1.10_error.txt

$ varnishd -F -f test.vcl 

Error:
Message from VCC-compiler:
Symbol not found: 'primary_director' (expected type BACKEND):
('test.vcl' Line 15 Pos 35)
    fallback_director.add_backend(primary_director);
----------------------------------################--

('test.vcl' Line 15 Pos 5) -- ('test.vcl' Line 15 Pos 34)
    fallback_director.add_backend(primary_director);
----##############################------------------

Running VCC-compiler failed, exited with 2
VCL compilation failed

varnish_6.4.0_error.txt

$ varnishd -F -f test.vcl

Error:
Message from VCC-compiler:
Expression has type INSTANCE, expected BACKEND
('/etc/varnish/test.vcl' Line 15 Pos 35) -- (Pos 50)
    fallback_director.add_backend(primary_director);
----------------------------------################--

Expected argument: BACKEND (unnamed argument)

While compiling function call:

('/etc/varnish/test.vcl' Line 15 Pos 5) -- (Pos 50)
    fallback_director.add_backend(primary_director);
----##############################################--

Running VCC-compiler failed, exited with 2
VCL compilation failed