The error appeared after upgrading ruby to 2.0.0-p451

There are two possible backtraces:

/home/apps/.rvm/rubies/ruby-2.0.0-p451/lib/ruby/2.0.0/set.rb:239→ []=
/home/apps/.rvm/rubies/ruby-2.0.0-p451/lib/ruby/2.0.0/set.rb:239→ add
mongo-1.9.2/lib/mongo/util/pool_manager.rb:233→ assign_primary
mongo-1.9.2/lib/mongo/util/pool_manager.rb:212→ block in initialize_pools
/home/apps/.rvm/rubies/ruby-2.0.0-p451/lib/ruby/2.0.0/set.rb:232→ each_key
/home/apps/.rvm/rubies/ruby-2.0.0-p451/lib/ruby/2.0.0/set.rb:232→ each
mongo-1.9.2/lib/mongo/util/pool_manager.rb:208→ initialize_pools

and

/home/apps/.rvm/rubies/ruby-2.0.0-p451/lib/ruby/2.0.0/set.rb:239→ []=
/home/apps/.rvm/rubies/ruby-2.0.0-p451/lib/ruby/2.0.0/set.rb:239→ add
mongo-1.9.2/lib/mongo/util/pool_manager.rb:249→ assign_secondary
mongo-1.9.2/lib/mongo/util/pool_manager.rb:215→ block in initialize_pools
/home/apps/.rvm/rubies/ruby-2.0.0-p451/lib/ruby/2.0.0/set.rb:232→ each_key
/home/apps/.rvm/rubies/ruby-2.0.0-p451/lib/ruby/2.0.0/set.rb:232→ each
mongo-1.9.2/lib/mongo/util/pool_manager.rb:208→ initialize_pools