Can anyone explain what's going wrong the `if' modifier when using
`defined?'

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craig@duncan:/usr/home/craig/ruby$ cat tst3.rb
#!/usr/bin/ruby

if not defined? i
  i = 0
end

p i += 1

j = 0 if not defined? j

p j += 1

craig@duncan:/usr/home/craig/ruby$ tst3.rb
1
./tst3.rb:11: undefined method `+' for nil (NameError)
craig@duncan:/usr/home/craig/ruby$