Gem::FilePermissionError on "rails:freeze:gems" task

My understanding is that the "rails:freeze:gems" task copies the Rails source into the application's vendor directory. Which is why I find the following error surprising and confusing, because the task should not involve writing to system files or directories:

% rake rails:freeze:gems (in /Users/tallen/apps/pnc) Freezing to the gems for Rails 2.3.5 rm -rf vendor/rails mkdir -p vendor/rails cd vendor/rails ERROR: While executing gem ... (Gem::FilePermissionError)     You don't have write permissions into the /opt/local/lib/ruby/gems/ 1.8 directory. rm -rf vendor/rails

Why does the task want to write to that directory? The output was identical with the --trace option.

Thomas