URL invoke Error

URL invoke Error

Hi there, 
I need help. 
I'm getting an error. 
"Given index 0 is greater than the list size Line:(18)" 
Essentially I'm trying to Get a user, and then update the same user. 
The list is to iterate through a list of users I have. 
Help appreciated! 

  1. payload = Map();
  2. payload.put("Active","true");
  3. user_name = input.Bulk_Remove_Teams1;
  4. userNameList = user_name.toList(",");
  5. for each  user_name in userNameList
  6. {
  7. headers = Map();
  8. headers.put("apikey","xxxxxxxxxxxxxxxxxxxxxxxxxxx");
  9. headers.put("Content-Type","application/json");
  10. get_user_by_username = "https://api.litmos.com.au/v1.svc/users?source=GET-USER-BY-USERNAME&format=json&search=" + user_name;
  11. //get_user_by_id =  "https://api.litmos.com.au/v1.svc/users/8_FZC7oILJQ1";
  12. user = invokeurl
  13. [
  14. url :get_user_by_username
  15. type :GET
  16. headers:headers
  17. ];
  18. user_id = user.get(0).get("Id");
  19. change_user = "https://api.litmos.com.au/v1.svc/users/" + user_id + "?source=changeuser";
  20. response = invokeurl
  21. [
  22. url :change_user
  23. type :PUT
  24. parameters:payload
  25. headers:headers
  26. ];
  27. }