Some sample code couldn't perform on Zorba normally

[lhchingit]

Hi,
I directly copy-and paste following sample code on Try Zorba but I got error.

let $person := {
  "name" : "Sarah",
  "age" : 13,
  "gender" : "female",
  "friends" : [ "Jim", "Mary", "Jennifer"]
}
return $person.friends[]

The error message showed (no URI):7,24: static error [err:XPST0003]: invalid expression: syntax error, unexpected "]". How could I fix this problem, thanks in advance.