Why this recursion example in ocaml doesn't work for negative number?

In the Think Ocaml book the author gave this example:

let rec countdown n =     if n <= 0 then       ( print_string "Blastoff!";         print_newline())     else ( print_int n; print_newline();        countdown (n-1); ());;

The function takes an int and returns a unit. However, the code suppose to work on negative numbers as mentioned in the book but it doesn't.The logic of the code seems fine and nothing wrong with it. I thought I would share it with people who know Ocaml to see what is wrong here.