Unable to make middleman-blog custom_collections work.

config.rb

blog.custom_collections = {
  topic: {
    link: 'topic/:topic/index.html',
    template: 'topics.html'
  }
}

stack trace

/Library/Ruby/Gems/2.0.0/gems/activesupport-3.2.16/lib/active_support/multibyte/unicode.rb:235:in `tidy_bytes': undefined method `unpack' for nil:NilClass (NoMethodError)
	from /Library/Ruby/Gems/2.0.0/gems/activesupport-3.2.16/lib/active_support/inflector/transliterate.rb:63:in `transliterate'
	from /Library/Ruby/Gems/2.0.0/gems/middleman-blog-3.5.1/lib/middleman-blog/uri_templates.rb:52:in `safe_parameterize'
	from /Library/Ruby/Gems/2.0.0/gems/middleman-blog-3.5.1/lib/middleman-blog/custom_pages.rb:25:in `link'
	from /Library/Ruby/Gems/2.0.0/gems/middleman-blog-3.5.1/lib/middleman-blog/custom_pages.rb:31:in `block in manipulate_resource_list'
	from /Library/Ruby/Gems/2.0.0/gems/middleman-blog-3.5.1/lib/middleman-blog/custom_pages.rb:30:in `each'
	from /Library/Ruby/Gems/2.0.0/gems/middleman-blog-3.5.1/lib/middleman-blog/custom_pages.rb:30:in `map'
	from /Library/Ruby/Gems/2.0.0/gems/middleman-blog-3.5.1/lib/middleman-blog/custom_pages.rb:30:in `manipulate_resource_list'
	from /Library/Ruby/Gems/2.0.0/gems/middleman-core-3.2.1/lib/middleman-core/sitemap/store.rb:217:in `block (2 levels) in ensure_resource_list_updated!'
	from /Library/Ruby/Gems/2.0.0/gems/middleman-core-3.2.1/lib/middleman-core/sitemap/store.rb:216:in `each'
	from /Library/Ruby/Gems/2.0.0/gems/middleman-core-3.2.1/lib/middleman-core/sitemap/store.rb:216:in `inject'
	from /Library/Ruby/Gems/2.0.0/gems/middleman-core-3.2.1/lib/middleman-core/sitemap/store.rb:216:in `block in ensure_resource_list_updated!'
	from /System/Library/Frameworks/Ruby.framework/Versions/2.0/usr/lib/ruby/2.0.0/monitor.rb:211:in `mon_synchronize'
	from /Library/Ruby/Gems/2.0.0/gems/middleman-core-3.2.1/lib/middleman-core/sitemap/store.rb:210:in `ensure_resource_list_updated!'
	from /Library/Ruby/Gems/2.0.0/gems/middleman-core-3.2.1/lib/middleman-core/sitemap/extensions/on_disk.rb:35:in `block in initialize'
	from /Library/Ruby/Gems/2.0.0/gems/middleman-core-3.2.1/lib/vendored-middleman-deps/hooks-0.2.0/lib/hooks.rb:53:in `instance_exec'
	from /Library/Ruby/Gems/2.0.0/gems/middleman-core-3.2.1/lib/vendored-middleman-deps/hooks-0.2.0/lib/hooks.rb:53:in `block in run_hook_for'
	from /Library/Ruby/Gems/2.0.0/gems/middleman-core-3.2.1/lib/vendored-middleman-deps/hooks-0.2.0/lib/hooks.rb:49:in `each'
	from /Library/Ruby/Gems/2.0.0/gems/middleman-core-3.2.1/lib/vendored-middleman-deps/hooks-0.2.0/lib/hooks.rb:49:in `run_hook_for'
	from /Library/Ruby/Gems/2.0.0/gems/middleman-core-3.2.1/lib/vendored-middleman-deps/hooks-0.2.0/lib/hooks.rb:107:in `run_hook'
	from /Library/Ruby/Gems/2.0.0/gems/middleman-core-3.2.1/lib/middleman-core/core_extensions/request.rb:51:in `inst'
	from /Library/Ruby/Gems/2.0.0/gems/middleman-core-3.2.1/lib/middleman-core/cli/build.rb:91:in `shared_instance'
	from /Library/Ruby/Gems/2.0.0/gems/middleman-core-3.2.1/lib/middleman-core/cli/build.rb:60:in `build'
	from /Library/Ruby/Gems/2.0.0/gems/thor-0.18.1/lib/thor/command.rb:27:in `run'
	from /Library/Ruby/Gems/2.0.0/gems/thor-0.18.1/lib/thor/invocation.rb:120:in `invoke_command'
	from /Library/Ruby/Gems/2.0.0/gems/thor-0.18.1/lib/thor.rb:363:in `dispatch'
	from /Library/Ruby/Gems/2.0.0/gems/thor-0.18.1/lib/thor/base.rb:439:in `start'
	from /Library/Ruby/Gems/2.0.0/gems/middleman-core-3.2.1/lib/middleman-core/cli.rb:77:in `method_missing'
	from /Library/Ruby/Gems/2.0.0/gems/thor-0.18.1/lib/thor/command.rb:29:in `run'
	from /Library/Ruby/Gems/2.0.0/gems/thor-0.18.1/lib/thor/command.rb:128:in `run'
	from /Library/Ruby/Gems/2.0.0/gems/thor-0.18.1/lib/thor/invocation.rb:120:in `invoke_command'
	from /Library/Ruby/Gems/2.0.0/gems/thor-0.18.1/lib/thor.rb:363:in `dispatch'
	from /Library/Ruby/Gems/2.0.0/gems/thor-0.18.1/lib/thor/base.rb:439:in `start'
	from /Library/Ruby/Gems/2.0.0/gems/middleman-core-3.2.1/lib/middleman-core/cli.rb:22:in `start'
	from /Library/Ruby/Gems/2.0.0/gems/middleman-core-3.2.1/bin/middleman:18:in `<top (required)>'
	from /usr/bin/middleman:23:in `load'
	from /usr/bin/middleman:23:in `<main>'

versions

**OS version.**

OS X Mavericks 10.9.1

**Ruby version.**

ruby 2.0.0p247 (2013-06-27 revision 41674) [universal.x86_64-darwin13]

(Standard Ruby version on Mavericks 10.9.1.)

**Gem versions.**

activesupport (3.2.16)
addressable (2.3.5)
atomic (1.1.14)
blankslate (2.1.2.4)
builder (3.2.2)
bundler (1.5.1)
CFPropertyList (2.2.0)
chunky_png (1.2.9)
classifier (1.3.4)
coffee-script (2.2.0)
coffee-script-source (1.6.3)
colorator (0.1)
commander (4.1.5)
compass (0.12.2)
em-websocket (0.5.0)
eventmachine (1.0.3)
execjs (1.4.0)
fast-stemmer (1.0.2)
ffi (1.9.3)
fssm (0.2.10)
haml (4.0.4)
highline (1.6.20)
hike (1.2.3)
http_parser.rb (0.5.3)
i18n (0.6.9)
jekyll (1.4.2)
kramdown (1.3.1, 1.3.0)
libxml-ruby (2.6.0)
liquid (2.5.4)
listen (1.3.1)
maruku (0.7.0)
middleman (3.2.1)
middleman-blog (3.5.1)
middleman-core (3.2.1)
middleman-deploy (0.1.4)
middleman-livereload (3.1.0)
middleman-sprockets (3.2.0)
multi_json (1.8.2)
net-sftp (2.1.2)
net-ssh (2.7.0)
nokogiri (1.5.6)
parslet (1.5.0)
posix-spawn (0.3.8)
ptools (1.2.2)
pygments.rb (0.5.4)
rack (1.5.2)
rack-livereload (0.3.15)
rack-test (0.6.2)
rb-fsevent (0.9.4)
rb-inotify (0.9.3)
rb-kqueue (0.2.0)
redcarpet (2.3.0)
safe_yam
l (0.9.7)
sass (3.2.13)
sprockets (2.10.1)
sprockets-helpers (1.0.1)
sprockets-sass (1.0.2)
sqlite3 (1.3.7)
susy (1.0.9)
thor (0.18.1)
thread_safe (0.1.3)
tilt (1.4.1)
toml (0.1.0)
tzinfo (1.1.0)
uglifier (2.1.2)
yajl-ruby (1.1.0)