kc3-lang/kc3

diff --git a/lib/kc3/0.1/kc3.facts b/lib/kc3/0.1/kc3.facts
index 7517b82..09302c2 100644
--- a/lib/kc3/0.1/kc3.facts
+++ b/lib/kc3/0.1/kc3.facts
@@ -105,3 +105,5 @@ add {KC3, :symbol, KC3.is_integer?}
 replace {KC3.is_integer?, :symbol_value, cfn Bool "tag_is_integer" (Tag)}
 add {KC3, :symbol, KC3.sysctl}
 replace {KC3.sysctl, :symbol_value, cfn Tag "kc3_sysctl" (Result, List)}
+add {KC3, :symbol, KC3.nil}
+replace {KC3.nil, :symbol_value, void}
diff --git a/libkc3/cfn.c b/libkc3/cfn.c
index 6b13bd5..8c1cb0d 100644
--- a/libkc3/cfn.c
+++ b/libkc3/cfn.c
@@ -106,6 +106,7 @@ s_tag * cfn_apply (s_cfn *cfn, s_list *args, s_tag *dest)
           if (! tag_to_ffi_pointer(&a->tag, cfn_arg_types->tag.data.sym,
                                    arg_pointers + i)) {
             err_puts("cfn_apply: tag_to_ffi_pointer 4");
+            err_stacktrace();
             assert(! "cfn_apply: tag_to_ffi_pointer 4");
             goto ko;
           }