using format() on lists produces --empty-list--

When combining usage of the format("%S, mylist) function with difference(), it produces a string -empty-list- that is likely a internal placeholder.
The list does not show this placeholder when iterated over nor when using the join() function.

Manual test case attached and below, with output.

<pre>
body common control
{
bundlesequence =>

{ "test" }

;
}

bundle agent test
{
vars:
"a" slist =>

{ "test", ".", "item", ".." };
"b" slist => { ".", ".." };

"diff_between_a_and_b" slist => difference(a, b);

"a_str" string => format("%S", a);
"b_str" string => format("%S", b);
"diff_str_format" string => format("%S", diff_between_a_and_b);
"diff_str_join" string => join( ",", diff_between_a_and_b);

reports:
agent::
"a: $(a_str)";
"b: $(b_str)";
"diff from format: $(diff_str_format)";
"diff from join: $(diff_str_join)";
"diff item: $(diff_between_a_and_b)";
}
</pre>


<pre>
[root@node001 tmp]# cf-agent -K test.cf
R: a: { "test", ".", "item", ".." }

R: b:

{ ".", ".." }

R: diff from format:

{ --empty-list--, "test", "item" }

R: diff from join: test,item
R: diff item: test
R: diff item: item
</pre>