exception with rails activesupport-3.1.0.rc4 - "incompatible character encodings: UTF-8 and ASCII-8BIT"

[trevor]

% gem rdoc -V --overwrite -v 3.1.0.rc4 activesupport
Installing RDoc documentation for activesupport-3.1.0.rc4...
rdoc --op /usr/local/Cellar/ruby/1.9.2-p180/lib/ruby/gems/1.9.1/doc/activesupport-3.1.0.rc4/rdoc --inline-source --line-numbers --fmt=html --template=hanna lib --title activesupport-3.1.0.rc4 Documentation --quiet
error while writing to #<File:classes/String.html>
ERROR:  While generating documentation for activesupport-3.1.0.rc4
... MESSAGE:   incompatible character encodings: UTF-8 and ASCII-8BIT
... RDOC args: --op /usr/local/Cellar/ruby/1.9.2-p180/lib/ruby/gems/1.9.1/doc//rdoc --inline-source --line-numbers --fmt=html --template=hanna lib --title activesupport-3.1.0.rc4 Documentation --quiet


% cat ~/.gemrc
gem: --no-ri
rdoc: --inline-source --line-numbers --fmt=html --template=hanna

% gem -v                            
1.8.5

% gem list -l hanna  

*** LOCAL GEMS ***

hanna (0.1.12)

% ruby -v
ruby 1.9.2p180 (2011-02-18 revision 30909) [x86_64-darwin10.7.0]

% rdoc --version
rdoc 3.8