no implicit conversion of nil into String (TypeError)

[improvethings]

$ uname -srm
Darwin 17.7.0 x86_64

$ redsnapper
/System/Library/Frameworks/Ruby.framework/Versions/2.3/usr/lib/ruby/2.3.0/open3.rb:199:in spawn': no implicit conversion of nil into String (TypeError) from /System/Library/Frameworks/Ruby.framework/Versions/2.3/usr/lib/ruby/2.3.0/open3.rb:199:in popen_run'
from /System/Library/Frameworks/Ruby.framework/Versions/2.3/usr/lib/ruby/2.3.0/open3.rb:95:in popen3' from /Library/Ruby/Gems/2.3.0/gems/redsnapper-0.5.0/lib/redsnapper.rb:47:in files'
from /Library/Ruby/Gems/2.3.0/gems/redsnapper-0.5.0/lib/redsnapper.rb:77:in files_to_extract' from /Library/Ruby/Gems/2.3.0/gems/redsnapper-0.5.0/lib/redsnapper.rb:86:in file_groups'
from /Library/Ruby/Gems/2.3.0/gems/redsnapper-0.5.0/lib/redsnapper.rb:101:in run' from /Library/Ruby/Gems/2.3.0/gems/redsnapper-0.5.0/bin/redsnapper:18:in <top (required)>'
from /usr/local/bin/redsnapper:22:in load' from /usr/local/bin/redsnapper:22:in

'