diff --git a/src/lib/charsio.pl b/src/lib/charsio.pl
index 3c5affa9..5ff611f6 100644
--- a/src/lib/charsio.pl
+++ b/src/lib/charsio.pl
@@ -260,18 +260,11 @@ chars_base64(Cs, Bs, Options) :-
         ;   domain_error(charset, Charset, chars_base64/3)
         ),
         (   var(Cs) ->
-            must_be_characters(Bs),
+            must_be(chars, Bs),
             '$chars_base64'(Cs, Bs, Padding, Charset)
-        ;   must_be_characters(Cs),
+        ;   must_be(chars, Cs),
             (   '$first_non_octet'(Cs, N) ->
                 domain_error(byte_char, N, chars_base64/3)
             ;   '$chars_base64'(Cs, Bs, Padding, Charset)
             )
         ).
-
-must_be_characters(Cs) :-
-        (   partial_string(Cs) ->
-            true
-        ;   must_be(list, Cs),
-            maplist(must_be(character), Cs)
-        ).
diff --git a/src/lib/crypto.pl b/src/lib/crypto.pl
index 3d19a72b..efc478ae 100644
--- a/src/lib/crypto.pl
+++ b/src/lib/crypto.pl
@@ -1,5 +1,5 @@
 /* - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
-   Written May 2020 by Markus Triska (triska@metalevel.at)
+   Written 2020, 2021 by Markus Triska (triska@metalevel.at)
    Part of Scryer Prolog.
 
    Predicates for cryptographic applications.
@@ -46,7 +46,6 @@
 :- use_module(library(format)).
 :- use_module(library(charsio)).
 :- use_module(library(si)).
-:- use_module(library(iso_ext), [partial_string/1]).
 
 /* - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
    hex_bytes(?Hex, ?Bytes) is det.
@@ -105,21 +104,12 @@ must_be_bytes(Bytes, Context) :-
 
 
 must_be_byte_chars(Chars, Context) :-
-        (   partial_string(Chars) ->
-            (   '$first_non_octet'(Chars, F) ->
-                domain_error(byte_char, F, Context)
-            ;   true
-            )
-        ;   must_be(list, Chars),
-            (   member(Char, Chars),
-                char_code(Char, Code),
-                \+ between(0, 255, Code) ->
-                domain_error(byte_char, Char, Context)
-            ;   true
-            )
+        must_be(chars, Chars),
+        (   '$first_non_octet'(Chars, F) ->
+            domain_error(byte_char, F, Context)
+        ;   true
         ).
 
-
 /* - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
    Cryptographically secure random numbers
    =======================================
@@ -620,11 +610,7 @@ encoding_chars(octet, Bs, Cs) :-
         ),
         must_be_byte_chars(Cs, crypto_encoding).
 encoding_chars(utf8, Cs, Cs) :-
-        (   partial_string(Cs) ->
-            true
-        ;   must_be(list, Cs),
-            maplist(must_be(character), Cs)
-        ).
+        must_be(chars, Cs).
 
 /* - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
    Digital signatures with Ed25519
diff --git a/src/lib/error.pl b/src/lib/error.pl
index a79bd560..d6bf8708 100644
--- a/src/lib/error.pl
+++ b/src/lib/error.pl
@@ -1,3 +1,8 @@
+/* - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
+   Written September 2018 by Markus Triska (triska@metalevel.at)
+   I place this code in the public domain. Use it in any way you want.
+- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - */
+
 :- module(error, [must_be/2,
                   can_be/2,
                   instantiation_error/1,
@@ -5,12 +10,6 @@
                   type_error/3
                   ]).
 
-/* - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
-   Written September 2018 by Markus Triska (triska@metalevel.at)
-   I place this code in the public domain. Use it in any way you want.
-- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - */
-
-
 /* - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
    must_be(Type, Term)
 
@@ -25,10 +24,12 @@
 
    Currently, the following types are supported:
 
-       - integer
        - atom
-       - list
        - boolean
+       - character
+       - chars
+       - integer
+       - list
 - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - */
 
 must_be(Type, Term) :-
@@ -45,10 +46,27 @@ must_be_(var, Term) :-
 must_be_(integer, Term) :- check_(integer, integer, Term).
 must_be_(atom, Term)    :- check_(atom, atom, Term).
 must_be_(character, T)  :- check_(error:character, character, T).
+must_be_(chars, Ls) :-
+        (   ground(Ls), '$is_partial_string'(Ls) ->
+            % The expected case (success) uses a very fast test.
+            % We cannot use partial_string/1 from library(iso_ext),
+            % because that library itself imports library(error).
+            true
+        ;   must_be(list, Ls),
+            all_characters(Ls)
+        ).
 must_be_(list, Term)    :- check_(error:ilist, list, Term).
 must_be_(type, Term)    :- check_(error:type, type, Term).
 must_be_(boolean, Term) :- check_(error:boolean, boolean, Term).
 
+% We cannot use maplist(must_be(character), Cs), because library(lists)
+% uses library(error), so importing it would create a cyclic dependency.
+
+all_characters([]).
+all_characters([C|Cs]) :-
+        must_be(character, C),
+        all_characters(Cs).
+
 check_(Pred, Type, Term) :-
         (   var(Term) -> instantiation_error(must_be/2)
         ;   call(Pred, Term) -> true
@@ -69,6 +87,7 @@ type(type).
 type(integer).
 type(atom).
 type(character).
+type(chars).
 type(list).
 type(var).
 type(boolean).
@@ -97,6 +116,7 @@ can_be(Type, Term) :-
 can_(integer, Term) :- integer(Term).
 can_(atom, Term)    :- atom(Term).
 can_(character, T)  :- character(T).
+can_(chars, Ls)     :- '$is_partial_string'(Ls).
 can_(list, Term)    :- list_or_partial_list(Term).
 can_(boolean, Term) :- boolean(Term).