apt-offline get : IndexError: list index out of range

I'm trying to upgrade a computer under Ubuntu 16 without internet from another computer under Ubuntu 18. Under the Ubuntu16 computer I did: $ sudo apt-offline set ~/offline-data.sig Then I switched to my Ubuntu18 with the .sig file and tried using apt-offline but got this error when trying to download the packages:

bash
$ apt-offline get -d ./OfflineUpgrade/ offline-data.sig
Fetching APT Data
Traceback (most recent call last):
  File "/usr/bin/apt-offline", line 28, in 
    main()
  File "/usr/lib/python3/dist-packages/apt_offline_core/AptOfflineCoreLib.py", line 2211, in main
    args.func(args)
  File "/usr/lib/python3/dist-packages/apt_offline_core/AptOfflineCoreLib.py", line 1135, in fetcher
    (ItemURL, ItemFile, ItemSize, ItemChecksum) = stripper(item)
  File "/usr/lib/python3/dist-packages/apt_offline_core/AptOfflineCoreLib.py", line 814, in stripper
    item1 = ''.join(item)
IndexError: list index out of range

My offline-data.sig looks like this:

none
'https://esm.ubuntu.com/infra/ubuntu/dists/xenial-infra-security/InRelease ' esm.ubuntu.com_infra_ubuntu_dists_xenial-infra-security_InRelease 0 
'https://esm.ubuntu.com/infra/ubuntu/dists/xenial-infra-security/main/binary-amd64/Packages.xz ' esm.ubuntu.com_infra_ubuntu_dists_xenial-infra-security_main_binary-amd64_Packages 0 
'https://esm.ubuntu.com/infra/ubuntu/dists/xenial-infra-security/main/binary-i386/Packages.xz ' esm.ubuntu.com_infra_ubuntu_dists_xenial-infra-security_main_binary-i386_Packages 0 
'https://esm.ubuntu.com/infra/ubuntu/dists/xenial-infra-security/main/binary-all/Packages.xz ' esm.ubuntu.com_infra_ubuntu_dists_xenial-infra-security_main_binary-all_Packages 0 
'https://esm.ubuntu.com/infra/ubuntu/dists/xenial-infra-security/main/i18n/Translation-fr_FR.xz ' esm.ubuntu.com_infra_ubuntu_dists_xenial-infra-security_main_i18n_Translation-fr%5fFR 0 
'https://esm.ubuntu.com/infra/ubuntu/dists/xenial-infra-security/main/i18n/Translation-fr.xz ' esm.ubuntu.com_infra_ubuntu_dists_xenial-infra-security_main_i18n_Translation-fr 0 
'https://esm.ubuntu.com/infra/ubuntu/dists/xenial-infra-security/main/i18n/Translation-en.xz ' esm.ubuntu.com_infra_ubuntu_dists_xenial-infra-security_main_i18n_Translation-en 0 
'https://esm.ubuntu.com/infra/ubuntu/dists/xenial-infra-security/main/dep11/Components-amd64.yml.xz ' esm.ubuntu.com_infra_ubuntu_dists_xenial-infra-security_main_dep11_Components-amd64.yml 0

How can I solve this?