Cannot install a particular gem with rubygems 1.5

# ruby --version
ruby 1.9.2p0 (2010-08-18 revision 29036) [i386-freebsd8]
# gem --version
1.5.0

If I manually download https://rubygems.org/downloads/bwkfanboy-1.2.8.gem file and run gem install -l /where/is/bwkfanboy-1.2.8.gem everything is OK, but if I try to install the gem from the network, e.g. gem install bwkfanboy I'm getting an error: undefined class/module YAML::PrivateType. Here is a backtrace:

# gem install -V bwkfanboy --backtrace
GET http://rubygems.org/latest_specs.4.8.gz
302 Found
GET http://production.s3.rubygems.org/latest_specs.4.8.gz
200 OK
ERROR:  While executing gem ... (ArgumentError)
    undefined class/module YAML::PrivateType
        /usr/local/lib/ruby/site_ruby/1.9/rubygems/specification.rb:289:in `load'
        /usr/local/lib/ruby/site_ruby/1.9/rubygems/specification.rb:289:in `_load'
        /usr/local/lib/ruby/site_ruby/1.9/rubygems/spec_fetcher.rb:124:in `load'
        /usr/local/lib/ruby/site_ruby/1.9/rubygems/spec_fetcher.rb:124:in `fetch_spec'
        /usr/local/lib/ruby/site_ruby/1.9/rubygems/spec_fetcher.rb:86:in `block in fetch_with_errors'
        /usr/local/lib/ruby/site_ruby/1.9/rubygems/spec_fetcher.rb:85:in `map'
        /usr/local/lib/ruby/site_ruby/1.9/rubygems/spec_fetcher.rb:85:in `fetch_with_errors'
        /usr/local/lib/ruby/site_ruby/1.9/rubygems/dependency_installer.rb:108:in `find_gems_with_sources'
        /usr/local/lib/ruby/site_ruby/1.9/rubygems/dependency_installer.rb:212:in `find_spec_by_name_and_version'
        /usr/local/lib/ruby/site_ruby/1.9/rubygems/dependency_installer.rb:244:in `install'
        /usr/local/lib/ruby/site_ruby/1.9/rubygems/commands/install_command.rb:120:in `block in execute'
        /usr/local/lib/ruby/site_ruby/1.9/rubygems/commands/install_command.rb:115:in `each'
        /usr/local/lib/ruby/site_ruby/1.9/rubygems/commands/install_command.rb:115:in `execute'
        /usr/local/lib/ruby/site_ruby/1.9/rubygems/command.rb:278:in `invoke'
        /usr/local/lib/ruby/site_ruby/1.9/rubygems/command_manager.rb:133:in `process_args'
        /usr/local/lib/ruby/site_ruby/1.9/rubygems/command_manager.rb:103:in `run'
        /usr/local/lib/ruby/site_ruby/1.9/rubygems/gem_runner.rb:63:in `run'
        /usr/local/bin/gem:21:in `main'