[luatex] Glue node without spec field?

[Manuel Pégourié-Gonnard]

Hi,

>From the LuaTeX manual, I'm inclined to think that a glue node always have a
spec field. However, experience shows that it's not true:

\directlua{%
assert(callback.register('post_linebreak_filter', function(head)
    local n = node.slide(head.list)
    assert(node.type(n.id) == 'glue', 'last node is not glue.')
    assert(n.spec, 'glue node without spec field')
end))}
Blah.
\bye

compile with luatex 0.50.0 gives an error:

error: <\directlua >:1: glue node (subtype 9) without spec field

Am I misunderstanding the manual, or is this a bug?

Thanks,
Manuel.