Problem with fill tool: I draw a circle with a brush, lets asume black color and after, I try to fill the circle. There are always some white pixel left at the inside side of the circle. I guess I make something bad, but what ??? I just want the circle be

I draw a circle with a brush, lets asume black color and after, I try to fill the circle. There are always some white pixel left at the inside side of the circle. I guess I make something bad, but what ???
I just want the circle be full black