Can't create custom art brush, drag and drop not working
Help, I'm kind of a noob. I'm trying to create my own art brush, I can't make it work. I was trying to drag and drop the object circled below to the brush panel on the right to make this object an art brush. 
But when I was finally dropping the object on that brush panel, it didn't work. My cursor just changed into a "circle with slash/no symbol" icon.

I don't know what I'm doing wrong here. Take note I have already "expanded" the object. I've also tried simpler objects like that thick straight line above, but it's also not turning into an art brush when I tried dropping it in the brush panel. Please help!
