Why can't i fill line tool in adobe animate with bucket

I drew a simple square in Adobe Animate. I want to make this square's inside red by using the paint bucket. This doesn't work.

If I draw the square and then use the paint bucket it works.

No I don't want to know about selecting the option to make the square inside red. I want in general to know how to fill any shape I make with the line tool with connected lines with the paint bucket. The fact that I can't is idiotic.