Ok I understood

issue with your code, closing (cannot use ==)

It was javascript, fixed.

Already reported in the post below.

error messages are swaped expected -> actual

.

Nope, there is nothing like that.

BTW, how do you know?

Quite obvious from the error message:

main.rb:15:in `block (2 levels) in <main>': undefined method `>' for nil:NilClass (NoMethodError)
	from main.rb:5:in `block in find_index'
	from main.rb:4:in `each'
	from main.rb:4:in `find_index'
	from main.rb:15:in `block in <main>'
	from /runner/frameworks/ruby/cw-2.rb:55:in `block in describe'
	from /runner/frameworks/ruby/cw-2.rb:46:in `measure'
	from /runner/frameworks/ruby/cw-2.rb:51:in `describe'
	from /runner/frameworks/ruby/cw-2.rb:202:in `describe'
	from main.rb:11:in `<main>'

Not a kata issue.

passed test, error at stderror. what's wrong? (ruby language)

Which language?

result comments for last tests are swaped: expected <-> actual

Read this

there must be wrong with the tests. all passed but something wrong still carries on. and make the whole test fail

Empty string isn't tested in Python, it should be added as a fixed test.