numberformat woes?

[Klaus major-k]

Hi friends,

this snippet:
...
set the numberformat to 0000
put EMPTY into tNextNr
if tNextNr = EMPTY then
  put 1 + 0 into tNewNext
else
  put tNextNr + 1 into tNewNext
end if
put tNewNext
...
-> 0001

However if I put this into a function:
function fNextNr
  set the numberformat to 0000
  put the cLetzteLaufendeNummer of this stack into tNextNr
  if tNextNr = EMPTY then
      put 1 + 0 into tNewNext
  else
      put tNextNr + 1 into tNewNext
  end if
  set the cLetzteLaufendeNummer of this stack to tNewNext
  return tNewNext
end fNextNr

tNewNext = 1 (If tNextNr = EMPTY, resp 2,3,4..., if not)?

What am I missing? 
LC 9.5, macOS 10.14.6

Thanks a lot in advance!


Best

Klaus
--
Klaus Major
https://www.major-k.de
klaus at major-k.de