DS is working on conditions top down.
The first condition that matches will be executed (and then it will stop and execute the next task).
If no condition matches it will execute the (default).
If no condition matches and default, or the matched condition, is empty you get your error.
So what happend:
- No condition matched and default was empty, or
- The condition that matched first was empty
This is quiet normal ... you probably even got a warning that you have an empty "default" condition when you assigned the job (this warning can be disabled in the settings).