diff --git a/src/lib/sgml.pl b/src/lib/sgml.pl
index 0289278e..a0370e8c 100644
--- a/src/lib/sgml.pl
+++ b/src/lib/sgml.pl
@@ -1,28 +1,30 @@
 /* - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
    Predicates for parsing HTML and XML documents.
-   Written June 2020 by Markus Triska (triska@metalevel.at)
+   Written 2020-2022 by Markus Triska (triska@metalevel.at)
    Part of Scryer Prolog.
 
    Currently, two predicates are provided:
 
-   -  load_html(+Source, -Es, +Options)
-   -  load_xml(+Source, -Es, +Options)
+     -  load_html(+Source, -Es, +Options)
+     -  load_xml(+Source, -Es, +Options)
 
    These predicates parse HTML and XML documents, respectively.
 
-   Source must be a stream, specified as stream(S), or a file,
-   specified as file(Name), where Name is a list of characters, or a
-   list of characters with the document contents.
+   Source must be one of:
+
+     - a list of characters with the document contents
+     - stream(S), specifying a stream S from which to read the content
+     - file(Name), where Name is a list of characters specifying a file name.
 
    Es is unified with the abstract syntax tree of the parsed document,
    represented as a list of elements where each is of the form:
 
-   * a list of characters, representing text
-   * element(Name, Attrs, Children)
-     - Name is the name of the tag
-     - Attrs is a list of Key=Value pairs:
-       Key is an atom, and Value is a list of characters
-     - Children is a list of elements as specified here.
+     * a list of characters, representing text
+     * element(Name, Attrs, Children)
+       - Name, an atom, is the name of the tag
+       - Attrs is a list of Key=Value pairs:
+         Key is an atom, and Value is a list of characters
+       - Children is a list of elements as specified here.
 
    Currently, Options are ignored. In the future, more options may be
    provided to control parsing.
@@ -61,21 +63,32 @@
 :- use_module(library(charsio)).
 
 load_html(Source, Es, Options) :-
+        must_be_source(Source, load_html/3),
+        must_be(list, Options),
         load_structure_(Source, Es, Options, html).
 load_xml(Source, Es, Options) :-
+        must_be_source(Source, load_xml/3),
+        must_be(list, Options),
         load_structure_(Source, Es, Options, xml).
 
+must_be_source(Source, Context) :-
+        (   var(Source) -> instantiation_error(Context)
+        ;   is_sgml_source(Source) -> true
+        ;   domain_error(sgml_source, Source, Context)
+        ).
+
+is_sgml_source(file(Fs)) :- must_be(chars, Fs).
+is_sgml_source(stream(_)).
+is_sgml_source([]).
+is_sgml_source([C|Cs]) :- must_be(chars, [C|Cs]).
+
 load_structure_([], [], _, _).
 load_structure_([C|Cs], [E], Options, What) :-
         load_(What, [C|Cs], E, Options).
 load_structure_(file(Fs), [E], Options, What) :-
-        must_be(list, Options),
-        must_be(list, Fs),
-        atom_chars(File, Fs),
-        once(phrase_from_file(seq(Cs), File)),
+        once(phrase_from_file(seq(Cs), Fs)),
         load_(What, Cs, E, Options).
 load_structure_(stream(Stream), [E], Options, What) :-
-        must_be(list, Options),
         get_n_chars(Stream, _, Cs),
         load_(What, Cs, E, Options).