Errors when a file has been deleted

[jamie]

When a file is staged for deletion (git rm $file), then rubocop -r rubocop_lineup issues the following error instead of running:

undefined method `+' for nil:NilClass
/Users/jamie/code/vpay/eft3/vendor/bundle/ruby/2.3.0/gems/rubocop_lineup-0.4.0/lib/rubocop_lineup/line_number_calculator.rb:11:in `git_line_summary_to_numbers'`

This also appears to be the case after committing the deleted file, so it's of ongoing relevance.