Auto-script backup tftp issue Fortigate 1500 5.6

Hi Everybody, I have an issue with my auto-script backup on Fortigate 1500D , version 5.6 Please find below the configured script.

# show system auto-script

config system auto-script

edit "backup-config"

set interval 86400

set repeat 0

set start auto

set script "execute backup full-config tftp dir/config.txt IP"

next

end When I try without the script on the cli, It works fine and properly.

 # execute backup full-config tftp dir/config.txt IP ....

Please wait...

Connect to tftp server IP ....

###

Send config file to tftp server OK.

  But after if I check the auto-script result config, I have one issue :

# execute auto-script result backup-config Script backup-config output: ########## script name: backup-config ##########

========== #1, 2017-10-18 16:12:57 ==========  $ execute backup full-config tftp dir/config.txt IP 7109: Unknown action 0 Command fail. Return code -1

Do you have any ideas to resolve it  ? Best Regards, Alex