Rails Ruby on Rails

Adding a locale fallback seems to break unit tests which check validation failures.

Part of app is localised, but the rest, including all the standard validation messages should fall back to english.

With the following code in application.rb
config.i18n.default_locale = :lv
config.i18n.fallbacks = {:lv => :en}

All the test lines with "assert something.invalid?" fail with error 'I18n::InvalidLocale: nil is not a valid locale'.
Trace(below) doesn't show anything useful.
Removing fallbacks by commenting the fallback line fixes the issue.

Command failed with status (1): [c:/Ruby/bin/ruby.exe -I"lib;test" "c:/Ruby...]
c:/Ruby/lib/ruby/gems/1.8/gems/rake-0.8.7/lib/rake.rb:995
c:/Ruby/lib/ruby/gems/1.8/gems/rake-0.8.7/lib/rake.rb:1010:in call'<br/> c:/Ruby/lib/ruby/gems/1.8/gems/rake-0.8.7/lib/rake.rb:1010:insh'
c:/Ruby/lib/ruby/gems/1.8/gems/rake-0.8.7/lib/rake.rb:1094:in sh'<br/> c:/Ruby/lib/ruby/gems/1.8/gems/rake-0.8.7/lib/rake.rb:1029:inruby'
c:/Ruby/lib/ruby/gems/1.8/gems/rake-0.8.7/lib/rake.rb:1094:in ruby'<br/> c:/Ruby/lib/ruby/gems/1.8/gems/railties-3.0.0/lib/rails/test_unit/testing.rake:26<br/> c:/Ruby/lib/ruby/gems/1.8/gems/rake-0.8.7/lib/rake.rb:1112:inverbose'
c:/Ruby/lib/ruby/gems/1.8/gems/railties-3.0.0/lib/rails/test_unit/testing.rake:11
c:/Ruby/lib/ruby/gems/1.8/gems/rake-0.8.7/lib/rake.rb:636:in call'<br/> c:/Ruby/lib/ruby/gems/1.8/gems/rake-0.8.7/lib/rake.rb:636:inexecute'
c:/Ruby/lib/ruby/gems/1.8/gems/rake-0.8.7/lib/rake.rb:631:in each'<br/> c:/Ruby/lib/ruby/gems/1.8/gems/rake-0.8.7/lib/rake.rb:631:inexecute'
c:/Ruby/lib/ruby/gems/1.8/gems/rake-0.8.7/lib/rake.rb:597:in invoke_with_call_chain'<br/> c:/Ruby/lib/ruby/1.8/monitor.rb:242:insynchronize'
c:/Ruby/lib/ruby/gems/1.8/gems/rake-0.8.7/lib/rake.rb:590:in invoke_with_call_chain'<br/> c:/Ruby/lib/ruby/gems/1.8/gems/rake-0.8.7/lib/rake.rb:583:ininvoke'
c:/Ruby/lib/ruby/gems/1.8/gems/rake-0.8.7/lib/rake.rb:2051:in invoke_task'<br/> c:/Ruby/lib/ruby/gems/1.8/gems/rake-0.8.7/lib/rake.rb:2029:intop_level'
c:/Ruby/lib/ruby/gems/1.8/gems/rake-0.8.7/lib/rake.rb:2029:in each'<br/> c:/Ruby/lib/ruby/gems/1.8/gems/rake-0.8.7/lib/rake.rb:2029:intop_level'
c:/Ruby/lib/ruby/gems/1.8/gems/rake-0.8.7/lib/rake.rb:2068:in standard_exception_handling'<br/> c:/Ruby/lib/ruby/gems/1.8/gems/rake-0.8.7/lib/rake.rb:2023:intop_level'
c:/Ruby/lib/ruby/gems/1.8/gems/rake-0.8.7/lib/rake.rb:2001:in run'<br/> c:/Ruby/lib/ruby/gems/1.8/gems/rake-0.8.7/lib/rake.rb:2068:instandard_exception_handling'
c:/Ruby/lib/ruby/gems/1.8/gems/rake-0.8.7/lib/rake.rb:1998:in run'<br/> c:/Ruby/lib/ruby/gems/1.8/gems/rake-0.8.7/bin/rake:31<br/> c:/Ruby/bin/rake:19:inload'
c:/Ruby/bin/rake:19