Assigning a list to a key of a dict

[Wells]

Why can't I do this?

teams = { "SEA": "Seattle Mariners" }
for team, name in teams.items():
        teams[team]["roster"] = ["player1", "player2"]

I get an error:

Traceback (most recent call last):
  File "./gamelogs.py", line 53, in <module>
    teams[team]["roster"] = ["player1", "player2"]
TypeError: 'str' object does not support item assignment

You can see that I am trying to make a key called "roster" for each
team item in the dictionary, which is a list of players.