a question about reading in alphabet letters

Hi,

I am practicing reading English sentences from a file, using get0/2. My
codes are as follows:

readInSentences :-
open('sentences.txt', read, Stream, [encoding(utf8)]),
readIn(Stream, List),
close(Stream),
write(List),nl,
put_list(List).

readIn(Stream, []) :-
get0(Stream, -1).
readIn(Stream, [Word|Tail]) :-
get0(Stream, Word),
readIn(Stream, Tail).

put_list([]).
put_list([H|T]) :-
put(H),
put_list(T).

I tried my codes and I got:

5 ?- readInSentences.
[104,115,105,32,32,111,107,10,104,116,105,32,32,101,46,87,32,97,32,114,116,32,105,104,116,101,112,110,-1]
hsi  ok
hti  e.W a rt ihtepn
ERROR: put/1: Type error: `character' expected, found `-1'

The three sentences to be read are: This is a book. (Enter) That is a pen.
(Enter). We can write with the pen.

Could someone tell me why some of the alphabetic letters are not read? I
use utf-8 because eventually I want to try my codes on Chinese texts.

Thank you!

Hunter Wu
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