Building Gaya theme gets undefined method `downcase' error

[vichargrave]

I forked and cloned your Gaya theme for Github websites then tried to build using rake preview which gives the following errors:

$ rake preview                                                                                                                                     ✹
bundle exec jekyll serve --watch --drafts --baseurl '' --config _config.yml,_config-dev.yml
Configuration file: _config.yml
Configuration file: _config-dev.yml
jekyll 3.2.1 | Error:  undefined method `downcase' for #<Hash:0x007fdc7823b408>
rake aborted!
Command failed with status (1): [bundle exec jekyll serve --watch --drafts ...]
/Users/vic/src/gaya/Rakefile:19:in `block in <top (required)>'
/Library/Ruby/Gems/2.0.0/gems/rake-11.3.0/exe/rake:27:in `<top (required)>'
Tasks: TOP => preview
(See full trace by running task with --trace)