I opened a private (whitelisted) server to play with my friends, in Minecraft snapshot 15w05b. I logged in successfully and then, I tried to do a tellraw command. I have experience with this kind of commands, so I got surpriser with the error message:
Invalid json: malformed JSON at line 1 column 3

I tried re-writing the command, bi¡ut it didn't work.
I did the command well, here is an example:
/tellraw @a

Sorry for my bad English, I'm Spanish.