Помогите понять и простить^W^W ошибку в Ruby

irb(main):005:0* a = 'sys-apps/sandbox-2.6'
=> "sys-apps/sandbox-2.6"
irb(main):006:0> version = '2.6'
=> "2.6"
irb(main):007:0> 
irb(main):008:0* a.match(/-#{version}$/)
=> #<MatchData "-2.6">
irb(main):009:0> a.slice!(/-#{version}$/, '')
IndexError: undefined group name reference: 
	from (irb):9:in `slice!'
	from (irb):9
	from /usr/bin/irb:12:in `<main>'
irb(main):010:0>