Fill Tool Isn't Filling

My Grease Pencil Fill Tool isn’t filling. I’ve tried to use it on a closed circle created with the Circle Tool, but it wouldn’t fill up all the way either. Either way, even if I had an opening, I’m pretty sure it would’ve just filled the entire screen up.

Am I doing something wrong? What do I need to actually do?

Never mind. It was because I didn’t select Solid Fill in the Material Properties tab.