Problem with optional argument to an option in libargtable2

/* Written by con-f-use - gmx d o t de */

#include <stdio.h>
#include <stdlib.h>
#include <argtable2.h>

int main(int argc, char** argv) {
    int i;
    struct arg_file *argOutput, *argFiles;

    argOutput = arg_file0("o", NULL, NULL,
        "File to write output to. Default is 'output.txt'. Omit whole option to discard all output"
    );
    argOutput->hdr.flag |= ARG_HASOPTVALUE;
    argOutput->filename[0] = "output.txt";
    argFiles = arg_filen(NULL, NULL, "INPUT-FILES", 1, argc-1, "Input files");
    struct arg_end *end = arg_end(5);

    void *argtable[] = { argOutput, argFiles, end };

    if( arg_nullcheck(argtable) != 0 ) exit(EXIT_FAILURE);

    if( arg_parse(argc,argv,argtable) != 0 ) {
        arg_print_errors(stderr, end, "Error");
        printf("\nUsage:\n%s", argv[0]);
        arg_print_syntax(stdout, argtable, "\n");
        arg_print_glossary_gnu(stdout, argtable);
        exit(EXIT_FAILURE);
    }

    for(i=0; i<argFiles->count; ++i)
        printf("inputfile[%i] = %s\n", i, argFiles->filename[i]);

    if( argOutput->count > 0 )
        printf("outputfile = %s\n", argOutput->filename[0] );
    else
        printf("Default value used for output-file.\n");

    return EXIT_SUCCESS;
}