Upcase does not work with Cyrillics

[pboettcher]

irb(main):001:0> require 'unicode_utils/upcase'
=> true
irb(main):002:0> puts UnicodeUtils.upcase('абвгдеёжзийклмнопрстуфхцчшщъыьэюя')
абвгде╤жзийклмноп└┴┬├─┼╞╟╚╔╩╦╠═╬╧
=> nil

Expected result is:
АБВГДЕЁЖЗИЙКЛМНОПРСТУФХЦЧШЩЪЫЬЭЮЯ

[ZaWertun]

Can't confirm for unicode_utils-1.4.0.