unlist errors on a nested list of empty lists

Reproducible example:

x <- list(list(list(), list()))
unlist(x)

*> Error in as.character.factor(x) : malformed factor*

The error comes from the line

structure(res, levels = lv, names = nm, class = "factor")

which is called because unlist() thinks that some entry is a factor, 
with NULL levels and NULL names.  It's not legal for a factor to have 
NULL levels.  Probably it should never get here; the earlier test

if (.Internal(islistfactor(x, recursive))) {

should have been false, and then the result would have been

.Internal(unlist(x, recursive, use.names))

(with both recursive and use.names being TRUE), which returns NULL.

Duncan Murdoch

What should happen:

unlist(x)

NULL

R.version
platform       x86_64-apple-darwin15.6.0
arch           x86_64
os             darwin15.6.0
system         x86_64, darwin15.6.0
status
major          3
minor          5.0
year           2018
month          04
day            23
svn rev        74626
language       R
version.string R version 3.5.0 (2018-04-23)
nickname       Joy in Playing