When looking into the `pdftocio` package I find the following piece of code
```
try:
with open_pdf(path_in) as doc:
if toc_file.isatty() or print_toc:
# no input from user, switch to output mode and extract the toc
# of pdf
toc = read_toc(doc)
if len(toc) == 0:
print("error: no table of contents found", file=sys.stderr)
sys.exit(1)
if readable:
print(pprint_toc(toc))
else:
print(dump_toc(toc), end="")
sys.exit(0)
# an input is given, so switch to input mode
toc = parse_toc(toc_file)
write_toc(doc, toc)
```
As from using `shell-command-to-string` we find that `len(toc)` must be 0 (it returns an empty string), so then it is probably the case that python determines that some input has been given.
So maybe, even though in the `call-process` the INFILE command is set to `nil`, still python might
determine that some kind of input is given.