stringout() problem with Maxima 5.15.0



I have the following maxima file

------ test.max -------------------
output_integer(z) := block(

        [tmp],

        tmp : file_output_append,
        file_output_append : true,

        stringout( "output.txt", "/* integer */" ),
        stringout( "output.txt", z ),

        file_output_append : tmp,

        return(z)

);

output_integer(1);
------ test.max -------------------


using my NEW Maxima installation (maxima -b test.max), I have

------------------------------------------------
Maxima 5.15.0 http://maxima.sourceforge.net
Using Lisp CLISP 2.45 (2008-05-15)
Distributed under the GNU Public License. See the file COPYING.
Dedicated to the memory of William Schelter.
The function bug_report() provides bug reporting information.
(%i1)                           batch(test.max)

batching /home/seirios/test.max
(%i2) output_integer(z) := block([tmp], tmp : file_output_append,
file_output_append : true, stringout("output.txt", "/* integer */"),
stringout("output.txt", z), file_output_append : tmp, return(z))
(%o2) output_integer(z) := block([tmp], tmp : file_output_append,
file_output_append : true, stringout("output.txt", "/* integer */"),
stringout("output.txt", z), file_output_append : tmp, return(z))
(%i3)                          output_integer(1)
Error in `stringout' attempt
#0: output_integer(z=1)(test.max line 7)
 -- an error.  To debug this try debugmode(true);
------------------------------------------------


whereas using my OLD Maxima installation (maxima -b test.max), I have

------------------------------------------------
Maxima 5.13.0 http://maxima.sourceforge.net
Using Lisp CLISP 2.41 (2006-10-13)
Distributed under the GNU Public License. See the file COPYING.
Dedicated to the memory of William Schelter.
This is a development version of Maxima. The function bug_report()
provides bug reporting information.
(%i1)                           batch(test.max)

batching /home/seirios/test.max
(%i2) output_integer(z) := block([tmp], tmp : file_output_append,
file_output_append : true, stringout("output.txt", "/* integer */"),
stringout("output.txt", z), file_output_append : tmp, return(z))
(%o2) output_integer(z) := block([tmp], tmp : file_output_append,
file_output_append : true, stringout("output.txt", "/* integer */"),
stringout("output.txt", z), file_output_append : tmp, return(z))
(%i3)                          output_integer(1)
(%o3)                                  1
------------------------------------------------

Andre