This fails only on Psych. On Syck, it works fine.

gistfile1.rb

require 'yaml'
str = <<-EOF
- foo
2012-06-04 12:02:16.979
EOF

str.to_yaml
#
# TypeError: can't convert nil into Integer
#  from /Users/hjust/.rvm/rubies/ruby-1.9.3-p327/lib/ruby/1.9.1/psych/scalar_scanner.rb:111:in `utc'
#  from /Users/hjust/.rvm/rubies/ruby-1.9.3-p327/lib/ruby/1.9.1/psych/scalar_scanner.rb:111:in `parse_time'
#  from /Users/hjust/.rvm/rubies/ruby-1.9.3-p327/lib/ruby/1.9.1/psych/scalar_scanner.rb:48:in `tokenize'
#  from /Users/hjust/.rvm/rubies/ruby-1.9.3-p327/lib/ruby/1.9.1/psych/visitors/yaml_tree.rb:242:in `visit_String'
#  from /Users/hjust/.rvm/rubies/ruby-1.9.3-p327/lib/ruby/1.9.1/psych/visitors/yaml_tree.rb:103:in `accept'
#  from /Users/hjust/.rvm/rubies/ruby-1.9.3-p327/lib/ruby/1.9.1/psych/visitors/yaml_tree.rb:67:in `push'
#  from /Users/hjust/.rvm/rubies/ruby-1.9.3-p327/lib/ruby/1.9.1/psych.rb:242:in `dump'
#  from /Users/hjust/.rvm/rubies/ruby-1.9.3-p327/lib/ruby/1.9.1/psych/core_ext.rb:14:in `psych_to_yaml'
#  from (irb):229
#  from /Users/hjust/.rvm/rubies/ruby-1.9.3-p327/bin/irb:16:in `<main>'