Ruby error UTF-8 to ASCII

I have a Ruby 1.9.2 install on Centos 5.5. Every time I install a gem, I get the error

ERROR:  While generating documentation for (gem)
... MESSAGE:   U+2018 from UTF-8 to US-ASCII
... RDOC args: lib History.txt README.txt

The gem still installs ok, but there are certain things I can't do. Like use SproutCore, for example. I get this little gem (pun intended).

sc-init xxxx
 ~ Created directory at xxxx
FATAL ~ invalid byte sequence in US-ASCII 

Anyone have any idea what's going on, and how to remedy it? Google isn't much help on th开发者_StackOverflow中文版is one.

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RDoc updated last week, and was supposed to help fix encoding problems.

Try gem update rdoc and see if that helps.