rdoc ignores --root option when generating ri documentation

I'm at the root of my project tree:

$ ls
bin  gem  lib  rdocs  test  vp.rb

I want to generate only the docs for the lib directory, ignoring the rest:

$ rm -r rdocs/ri
$ rdoc --root=lib --format=ri --op=rdocs/ri
Parsing sources...
100% [15/15]  vp.rb

Generating RI format into /home/FISRONA/gitwrk/vp5/rdocs/ri...

 Files:      15

Classes:     1 ( 1 undocumented)
Modules:     1 ( 0 undocumented)
Constants:   5 ( 3 undocumented)
Attributes:  1 ( 0 undocumented)
Methods:    18 ( 6 undocumented)

Total:      26 (10 undocumented)
 61.54% documented

Elapsed: 0.5s

From this output, we can already see that rdoc also parses vp.rb, which is in the working directory, and NOT in the directory specified by the --root option. Indeed, inspecting the genereated directory rdocs/ri, I can see that it also has parsed the files in the directory 'test' (for instance).

rdoc 4.2.0 (Cygwin)

Start with the rdoc command and reproduce the reported rdoc 4.2.0 invocation using --root=lib, --format=ri, and --op=rdocs/ri. Compare the parsed files with the project tree, especially vp.rb and test; done means only documentation from lib is generated.