Rvm does not find Ruby installed with macports

(Note: This topic was already posted in the macports mailing list. I’m
reposting it here, because I didn’t get any solution yet).

On my Snow Leopard, I have Ruby 1.8.7 (which came with the Mac), Ruby
1.9.1 (which I have installed with rvm), and Ruby 2.1.1, which I have
installed using macports afterwards. The respective pathes are:

/usr/bin/ruby # 1.8.7
$HOME/.rvm/rubies/ruby-1.9.1-p431/bin/ruby # 1.9.1
/opt/local/bin/ruby2.1 # 2.1.1

Now when I do a

    rvm list

I get the output:

    rvm rubies

macruby-0.12 [ i686 ]
=* ruby-1.9.1-p431 [ i386 ]

According to my understanding of rvm, it should honour the macport
installation, but it doesn’t. Why?