Help with Encoding

[greigwise]

I'm getting this with reverse_coverage when trying to save out the index:

Encoding::CompatibilityError:
incompatible character encodings: UTF-8 and ASCII-8BIT
(erb):34:in block (2 levels) in format' (erb):33:in each'
(erb):33:in block in format' /home/rof/cache/bundler/ruby/2.6.0/gems/reverse_coverage-0.1.1/lib/reverse_coverage/formatters/html/formatter.rb:21:in block in format'
/home/rof/cache/bundler/ruby/2.6.0/gems/reverse_coverage-0.1.1/lib/reverse_coverage/formatters/html/formatter.rb:20:in open' /home/rof/cache/bundler/ruby/2.6.0/gems/reverse_coverage-0.1.1/lib/reverse_coverage/formatters/html/formatter.rb:20:in format'
./spec/spec_helper.rb:87:in `block (2 levels) in <top (required)>'

Can anyone offer any advice on how to fix this?

Thanks,
Greig Wise