"middleman build --verbose" error

gistfile1.txt

error  build/2015/index.html
undefined local variable or method `month' for #<Middleman::Application:0x70289256449720>
/Users/hoge/.rbenv/versions/2.1.2/lib/ruby/gems/2.1.0/gems/middleman-core-3.4.0/lib/middleman-core/configuration.rb:78:in `method_missing'
/Users/hoge/blog/webfood/source/calendar.slim:1:in `block in singleton class'
/Users/hoge/blog/webfood/source/calendar.slim:-17:in `instance_eval'
/Users/hoge/blog/webfood/source/calendar.slim:-17:in `singleton class'
/Users/hoge/blog/webfood/source/calendar.slim:-19:in `__tilt_70289245756400'
/Users/hoge/.rbenv/versions/2.1.2/lib/ruby/gems/2.1.0/gems/tilt-1.4.1/lib/tilt/template.rb:170:in `call'
/Users/hoge/.rbenv/versions/2.1.2/lib/ruby/gems/2.1.0/gems/tilt-1.4.1/lib/tilt/template.rb:170:in `evaluate'
/Users/hoge/.rbenv/versions/2.1.2/lib/ruby/gems/2.1.0/gems/tilt-1.4.1/lib/tilt/template.rb:103:in `render'
/Users/hoge/.rbenv/versions/2.1.2/lib/ruby/gems/2.1.0/gems/middleman-core-3.4.0/lib/middleman-core/renderers/slim.rb:6:in `render'
/Users/hoge/.rbenv/versions/2.1.2/lib/ruby/gems/2.1.0/gems/padrino-helpers-0.12.5/lib/padrino/rendering.rb:16:in `render'
/Users/hoge/.rbenv/versions/2.1.2/lib/ruby/gems/2.1.0/gems/middleman-core-3.4.0/lib/middleman-core/core_extensions/rendering.rb:313:in `render_individual_file'
/Users/hoge/.rbenv/versions/2.1.2/lib/ruby/gems/2.1.0/gems/middleman-core-3.4.0/lib/middleman-core/core_extensions/rendering.rb:181:in `_render_with_all_renderers'
/Users/hoge/.rbenv/versions/2.1.2/lib/ruby/gems/2.1.0/gems/middleman-core-3.4.0/lib/middleman-core/core_extensions/rendering.rb:149:in `render_template'
/Users/hoge/.rbenv/versions/2.1.2/lib/ruby/gems/2.1.0/gems/middleman-core-3.4.0/lib/middleman-core/sitemap/resource.rb:127:in `block in render'
/Users/hoge/.rbenv/versions/2.1.2/lib/ruby/gems/2.1.0/gems/activesupport-4.2.4/lib/active_support/notifications.rb:166:in `instrument'
/Users/hoge/.rbenv/versions/2.1.2/lib/ruby/gems/2.1.0/gems/middleman-core-3.4.0/lib/middleman-core/util.rb:41:in `instrument'
/Users/hoge/.rbenv/versions/2.1.2/lib/ruby/gems/2.1.0/gems/middleman-core-3.4.0/lib/middleman-core/application.rb:241:in `instrument'
/Users/hoge/.rbenv/versions/2.1.2/lib/ruby/gems/2.1.0/gems/middleman-core-3.4.0/lib/middleman-core/sitemap/resource.rb:14:in `instrument'
/Users/hoge/.rbenv/versions/2.1.2/lib/ruby/gems/2.1.0/gems/middleman-core-3.4.0/lib/middleman-core/sitemap/resource.rb:100:in `render'
/Users/hoge/.rbenv/versions/2.1.2/lib/ruby/gems/2.1.0/gems/middleman-core-3.4.0/lib/middleman-core/core_extensions/request.rb:260:in `process_request'
/Users/hoge/.rbenv/versions/2.1.2/lib/ruby/gems/2.1.0/gems/middleman-core-3.4.0/lib/middleman-core/core_extensions/request.rb:210:in `block in call!'
/Users/hoge/.rbenv/versions/2.1.2/lib/ruby/gems/2.1.0/gems/middleman-core-3.4.0/lib/middleman-core/core_extensions/request.rb:209:in `catch'
/Users/hoge/.rbenv/versions/2.1.2/lib/ruby/gems/2.1.0/gems/middleman-core-3.4.0/lib/middleman-core/core_extensions/request.rb:209:in `call!'
/Users/hoge/.rbenv/versions/2.1.2/lib/ruby/gems/2.1.0/gems/middleman-core-3.4.0/lib/middleman-core/core_extensions/request.rb:195:in `call'
/Users/hoge/.rbenv/versions/2.1.2/lib/ruby/gems/2.1.0/gems/rack-1.6.4/lib/rack/urlmap.rb:66:in `block in call'
/Users/hoge/.rbenv/versions/2.1.2/lib/ruby/gems/2.1.0/gems/rack-1.6.4/lib/rack/urlmap.rb:50:in `each'
/Users/hoge/.rbenv/versions/2.1.2/lib/ruby/gems/2.1.0/gems/rack-1.6.4/lib/rack/urlmap.rb:50:in `call'
/Users/hoge/.rbenv/versions/2.1.2/lib/ruby/gems/2.1.0/gems/middleman-core-3.4.0/lib/middleman-more/extensions/minify_javascript.rb:49:in `call'
/Users/hoge/.rbenv/versions/2.1.2/lib/ruby/gems/2.1.0/gems/middleman-core-3.4.0/lib/middleman-more/extensions/minify_css.rb:54:in `call'
/Users/hoge/.rbenv/versions/2.1.2/lib/ruby/gems/2.1.0/gems/middleman-minify-html-3.1.1/lib/middleman-minify-html/vendor/htmlcompressor-0.0.6/lib/htmlcompressor/rack.rb:36:in `call'
/Users/hoge/.rbenv/versions/2.1.2/lib/ruby/gems/2.1.0/gems/rack-1.6.4/lib/rack/head.rb:13:in `call'
/Users/hoge/.rbenv/versions/2.1.2/lib/ruby/gems/2.1.0/gems/rack-1.6.4/lib/rack/lint.rb:49:in `_call'
/Users/hoge/.rbenv/versions/2.1.2/lib/ruby/gems/2.1.0/gems/rack-1.6.4/lib/rack/lint.rb:37:in `call'
/Users/hoge/.rbenv/versions/2.1.2/lib/ruby/gems/2.1.0/gems/rack-1.6.4/lib/rack/builder.rb:153:in `call'
/Users/hoge/.rbenv/versions/2.1.2/lib/ruby/gems/2.1.0/gems/rack-test-0.6.3/lib/rack/mock_session.rb:30:in `request'
/Users/hoge/.rbenv/versions/2.1.2/lib/ruby/gems/2.1.0/gems/rack-test-0.6.3/lib/rack/test.rb:244:in `process_request'
/Users/hoge/.rbenv/versions/2.1.2/lib/ruby/gems/2.1.0/gems/rack-test-0.6.3/lib/rack/test.rb:58:in `get'
/Users/hoge/.rbenv/versions/2.1.2/lib/ruby/gems/2.1.0/gems/middleman-core-3.4.0/lib/middleman-core/cli/build.rb:257:in `render_to_file'
/Users/hoge/.rbenv/versions/2.1.2/lib/ruby/gems/2.1.0/gems/middleman-core-3.4.0/lib/middleman-core/cli/build.rb:223:in `build_resource'
/Users/hoge/.rbenv/versions/2.1.2/lib/ruby/gems/2.1.0/gems/middleman-core-3.4.0/lib/middleman-core/cli/build.rb:215:in `each'
/Users/hoge/.rbenv/versions/2.1.2/lib/ruby/gems/2.1.0/gems/middleman-core-3.4.0/lib/middleman-core/cli/build.rb:215:in `execute!'
/Users/hoge/.rbenv/versions/2.1.2/lib/ruby/gems/2.1.0/gems/middleman-core-3.4.0/lib/middleman-core/cli/build.rb:128:in `invoke!'
/Users/hoge/.rbenv/versions/2.1.2/lib/ruby/gems/2.1.0/gems/thor-0.19.1/lib/thor/actions.rb:94:in `action'
/Users/hoge/.rbenv/versions/2.1.2/lib/ruby/gems/2.1.0/gems/middleman-core-3.4.0/lib/middleman-core/cli/build.rb:70:in `build'
/Users/hoge/.rbenv/versions/2.1.2/lib/ruby/gems/2.1.0/gems/thor-0.19.1/lib/thor/command.rb:27:in `run'
/Users/hoge/.rbenv/versions/2.1.2/lib/ruby/gems/2.1.0/gems/thor-0.19.1/lib/thor/invocation.rb:126:in `invoke_command'
/Users/hoge/.rbenv/versions/2.1.2/lib/ruby/gems/2.1.0/gems/thor-0.19.1/lib/thor.rb:359:in `dispatch'
/Users/hoge/.rbenv/versions/2.1.2/lib/ruby/gems/2.1.0/gems/thor-0.19.1/lib/thor/base.rb:440:in `start'
/Users/hoge/.rbenv/versions/2.1.2/lib/ruby/gems/2.1.0/gems/middleman-core-3.4.0/lib/middleman-core/cli.rb:72:in `method_missing'
/Users/hoge/.rbenv/versions/2.1.2/lib/ruby/gems/2.1.0/gems/thor-0.19.1/lib/thor/command.rb:29:in `run'
/Users/hoge/.rbenv/versions/2.1.2/lib/ruby/gems/2.1.0/gems/thor-0.19.1/lib/thor/command.rb:126:in `run'
/Users/hoge/.rbenv/versions/2.1.2/lib/ruby/gems/2.1.0/gems/thor-0.19.1/lib/thor/invocation.rb:126:in `invoke_command'
/Users/hoge/.rbenv/versions/2.1.2/lib/ruby/gems/2.1.0/gems/thor-0.19.1/lib/thor.rb:359:in `dispatch'
/Users/hoge/.rbenv/versions/2.1.2/lib/ruby/gems/2.1.0/gems/thor-0.19.1/lib/thor/base.rb:440:in `start'
/Users/hoge/.rbenv/versions/2.1.2/lib/ruby/gems/2.1.0/gems/middleman-core-3.4.0/lib/middleman-core/cli.rb:20:in `start'
/Users/hoge/.rbenv/versions/2.1.2/lib/ruby/gems/2.1.0/gems/middleman-core-3.4.0/bin/middleman:18:in `<top (required)>'
/Users/hoge/.rbenv/versions/2.1.2/bin/middleman:23:in `load'
/Users/hoge/.rbenv/versions/2.1.2/bin/middleman:23:in `<main>'