Rubygem problems (maybe just on MacOS 10.4)

It looks like there is a problem (I don’t know if it is Mac specific -
although I do not think so) with gem and proxies:

meisterbrau:~ riva$ gem update --http-proxy
ERROR: While executing gem … (NoMethodError)
undefined method `[]=’ for #Gem::ConfigFile:0x1087518

I have a Macports installation

meisterbrau:~ riva$ ruby -v
ruby 1.8.5 (2006-12-25 patchlevel 12) [i686-darwin8.8.2]
meisterbrau:~ riva$ gem -v

Any ideas?