Flatten, question

Everybody,
is somebody here who can help me in this small algorithm.
Why do I get
   atom_concat/3: Type error: `atomic' expected, found `[g]'
   atom_concat/3: Type error: `atomic' expected, found `[e]'
in the line where flatten1(...) is called with the list of a,b,c...
What's wrong with that... hmm? I found it in the Prolog book of Bratko, 
excercise 3.11
I am using SWI-Prolog 5.6
Regards,


flatten1([Head|Tail], List) :-
     flatten1(Head, FlatHead),
     flatten1(Tail, FlatTail),
     concat(FlatHead,FlatTail, List).

flatten1([], []).
flatten1(X, [X]).

:- flatten1([a,b,c,e,f,g], List),
    write('Flat List:'), write(List).

:- flatten1([a,b,[c,e],f,g], List),
    write('Flat List:'), write(List).