Python syntax error "cannot concatenate 'str' and 'NoneType' objects"

I found this script on the internet: http://ift.tt/1cCahwS

Any idea why I am getting the following error message? It looks like a syntax error. I am still new to Python, so please help me point out what the problem is and how to fix it.

[root@ip-172-31-18-97 tmppython]# python snapshot.py
INFO:root:Finding snapshot for tag...
Traceback (most recent call last):
  File "snapshot.py", line 164, in <module>
    main()
  File "snapshot.py", line 30, in main
    snapshot = find_snapshot(args.tag_name, args.tag_value)
  File "snapshot.py", line 47, in find_snapshot
    snapshots = conn.get_all_snapshots(filters={'tag:' + tag_name: tag_value})
TypeError: cannot concatenate 'str' and 'NoneType' objects