Publisher on licode gets a "onaddstream" event triggered (with a black screen)


On my client javascript library when I link the publisher to the
erizo_controller (with the peer connection) it triggers the "onaddstream"
event on my client side. Is the publisher supposed to be a send only? The
stream added contains one audio and one video but it’s all black (and
silent). Should I make something special on my sdp or when I create the
peer connection? Or is it normal and the erizo_controller should take care
of telling the user to NOT add this stream?