[Grammar-Kit] preview bug?

I'm playing with the Grammar-Kit and I can't understand preview behavior for the following simple grammar:

```

{
  tokens=[
    id = 'regexp:(_|\p{Alpha})\w*'
    space='regexp:\s+'
    // FOO = 'foo'
  ]
}

root ::= 'foo' id

```

Sample input: `foo foo`. Works until you uncomment `FOO` token definition, and then it says `expected foo got 'foo'` on the first input token. Is it a bug or I'm missing something?