Tests failing on Acronym exercise

Hi,

I’ve got this code:

abbreviate(Sentence, Acronym) :-
    split_string(Sentence, "- ", " \t\n", PhraseList),
    maplist(remove_non_alphanumeric, PhraseList, CleanList),
    maplist(first_char, CleanList, AcronymList),
    atomic_list_concat(AcronymList, AcronymString),
    upcase_atom(AcronymString, Acronym).

remove_non_alphanumeric(String, CleanString) :-
    atom_chars(String, Chars),
    include(alpha_numeric_char, Chars, AlphaNumericChars),
    atom_chars(CleanString, AlphaNumericChars).

alpha_numeric_char(Char) :-
    char_type(Char, alpha);
    char_type(Char, digit).

first_char(String, Char) :-
    atom_chars(String, [Char|_]).

When I test it interactively it works. However when I run the test, it fails with errors similar to:

ERROR: 
    [[ EXCEPTION while printing message url('/mnt/exercism-iteration/acronym_tests.plt':9)
       with arguments []:
       raised: type_error(text,url('/mnt/exercism-iteration/acronym_tests.plt':9))
    ]]
:
	test basic: failed

I’m completely new to Prolog but do have lots of experience with other programming likes like Python. Thanks in advance for your help on this.