Byebug不更新Ubuntu上的局部变量

# please_just_work.rb
require 'bundler/setup'
Bundler.require

byebug
puts "a = #{a}"

在byebug会话中,我设置了 a ,但尚未定义:

[1, 5] in /home/paper/tmp/debug.rb
   1: require 'bundler/setup'
   2: Bundler.require
   3: 
   4: byebug
=> 5: puts "a = #{a}"
(byebug) a = 1
1
(byebug) continue
Traceback (most recent call last):
please_just_work.rb:5:in `<main>': undefined local variable or method `a' for main:Object (NameError)

我正在运行ubuntu18.04