assertion failure in safePrinter

[treinen]

when I compile the following program

let filename = Sys.argv.(1)
let ast = Morsmall.parse_file filename
let _ = Morsmall.pp_print_safe Format.std_formatter ast

on the file consisting of one line

{ echo "hello world";}

then I get

{ echo Fatal error: exception File "src/safePrinter.ml", line 37, characters 5-11: Assertion failed

It works when I remove the quotes in the input.