adobe illustrator - How to fill in areas surrounded by line segments?

I have a rectangular shape and I added an "extension area" to it with several line segments and closed it back to the rectangle.

Specifically, I clicked on an anchor point off a corner of the rectangle drew several line segments off this point and connecting each by the anchor point of each segment. I drew and closed off this shape by drawing the line segment back to the rectangle's anchor point.

How do I fill in this new area with a color?

In the example below I'd like to fill in the "polygon shape" that's extended off of the rectangle with some non-white color. When I select the rectangle and polygon, I get a question mark: "?" in the fill area.