Rdoc & hanna: readme.md not included in doc?

When having a readme file named “readme.md” and running
rdoc -f hanna ./readme.md
the readme file is not included in rdoc-documentation. When renaming
“readme.md” to “readme.txt”, it is included . Am I doing something
wrong?

Using:
Windows7
ruby 1.9.3p448 (2013-06-27) [i386-mingw32]
rdoc 3.12.2 (hanna doesn’t support rdoc 4)
hanna is hanna-nouveau 0.2.7

You can try it like this:

cd \lib\ruby\gems\1.9.1\gems\bundler-1.3.5
rdoc -f hanna --main ./README.md ./README.md ./lib

=> Created documentation will not show README.md

(Be sure that hanna is used and not, for example, darkfish.)

Copy README.md to README.txt
rdoc -f hanna --main ./README.txt ./README.txt ./lib

=> Created documentation will show README.txt

Kind regards,
Axel