Ruby comes with a built-in REPL, which is quite handy.
Your challenge is to crash it in the least amount of code!
The definition of "crash" is "make it exit in an unintended way." This means
irb_quit, et. al. are not valid answers.
Furthermore, you may not cause any side-effect to any other part of the system. For example,
`rm -rf /` is not valid either.
Any version 1.9.3 or above is valid. If your code only works on a specific version of Ruby, you may specify that in the answer.
The final restriction is that you may not rely on any gems.
This is code-golf, so the shortest code that crashes IRB will win!