KeyNotFoundException: The given key was not present in the dictionary

Hello, so basically am new to unity and specially for uNet, I wanted to create a multiplayer game, I followed brackeys tutorials, It was going so good until I reached episode 7, In there When he did the damage part I did what he did, but then I got the following error : KeyNotFoundException: The given key was not present in the dictionary, I searched On internet and even went to the video comment section but no body talked about that, Hopefully I find a way to get rid of this, and here is the code problem:

    public static Player GetPlayer (string _playerID)
     {
         return players[_playerID];           // When I click the error it takes me to this line.
     }