Composition is still transparent even though I choose black?

I am quite new to After Effects and I have looked at many tutorials but for some reason my composition colour will not change, it just stays transparent.

As you can see my background colour is set as black. However, once I hit "ok", this happens..

I don't know if this is a bug or a glitch but no matter how many times I try it doesn't work. I even try changing the background colour to red, blue, etc.. and it still comes out like this?