Cannot append to a zip file

hi,
I'm trying to create a zip file with two input files.

The first 'zip file' step works fine, but the second 'zip file' step even with the option
'if zip File exists' set to 'Append file to the existing'
fails with a 'Could not create file XXXXXX , exception:URI is not absolute'.

Tested all I am able to imagine but no way: if the zip file exists I canot append.

What am I doing wrong?

Thks