[racket] Help with Scribble % DrRacket

 Trying to write documentation

---
#lang scribble/manual 
@(require (for-label racket)) 

@title{GObject Introspection} 

This is Gobject FFI. 

Usage example: 

@racketblock[ 
(gi-ffi gtk "Gtk") 
(let 
  ([window (gtk 'window 'new)]) (window 'show)) 
]

---

DrRacket writes in status line: +: contract violation expected: number? given: #f argument position: 1st other arguments...: 0

I push "Check syntax". The window hangs. To be correct, not window, but tab with the document. I still can work with other tabs.
Racket 5.3.4, Linux
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