jruby-9.0.0.0 :007 > x = "joe"
=> "joe"
jruby-9.0.0.0 :008 > y = "joe/⸀䐀攀氀攀琀攀䴀攀/fred"
=> "joe/⸀䐀攀氀攀琀攀䴀攀/fred"
jruby-9.0.0.0 :009 > Pathname.new(y).relative_path_from(Pathname.new(x))
TypeError: no implicit conversion from nil to integer
from org/jruby/RubyString.java:2870:in []' from /services/medusa/.rvm/rubies/jruby-9.0.0.0/lib/ruby/stdlib/pathname.rb:48:inchop_basename'
from /services/medusa/.rvm/rubies/jruby-9.0.0.0/lib/ruby/stdlib/pathname.rb:113:in cleanpath_aggressive' from /services/medusa/.rvm/rubies/jruby-9.0.0.0/lib/ruby/stdlib/pathname.rb:91:incleanpath'
from /services/medusa/.rvm/rubies/jruby-9.0.0.0/lib/ruby/stdlib/pathname.rb:493:in relative_path_from' from (irb):9:in'
from org/jruby/RubyKernel.java:979:in eval' from org/jruby/RubyKernel.java:1292:inloop'
from org/jruby/RubyKernel.java:1099:in catch' from org/jruby/RubyKernel.java:1099:incatch'
from /services/medusa/.rvm/rubies/jruby-9.0.0.0/bin/irb:13:in `'

Here's the MRI version, which works:

2.2.3 :006 > x = "joe"
=> "joe"
2.2.3 :007 > y = "joe/⸀䐀攀氀攀琀攀䴀攀/fred"
=> "joe/⸀䐀攀氀攀琀攀䴀攀/fred"
2.2.3 :008 > Pathname.new(y).relative_path_from(Pathname.new(x))
=> #Pathname:⸀䐀攀氀攀琀攀䴀攀/fred

I discovered this at the same time as issue #3392. They appear to be different problems, though.