13w05b Server @p argument for join team isn't working

While playing on my redstone testing server, I have run into a problem. When trying to put the nearest player on to a team using command blocks, this will not change the nearest player's current team. The command is /scoreboard teams join Yellow @p This would not work. I tried /scoreboard teams join Yellow Willco2 This command did work. I am using 13w05b on the correct version server.