Cannot teleport to an Entity when the Player is included in the command

I have recently discovered that I cannot use the teleport command on a player in a command block. I type: /tp @a @e in the command block. The command block output will then tell me: The entity UUID provided is in an invalid format When I type it in my chat bar, I can execute it if I type it like this: /tp @e. If I use the way listed above, I get the same UUID error.