Tripple tilde does not work

Markdown allows two styles for code blocks:

  - triple backtick: `` ``` ``
  - triple tilde: `~~~`

Triple backtick works fine:
~~~
```{scheme}
(display "works fine\n")
```
~~~

But triple tilde does not work:
```
~~~{scheme}
(display "does not work\n")
~~~
```