Shapes in Illustrator don't fill to the edges

I am having trouble in 2018CC Illustrator where I draw a shape with the pen tool and then when i zoom in really close, the colour doesn't fill to the edge.

I've attached a couple of photos so you can see what I mean.

The blue line is the pen tool's shape I have drawn, and the black edge is the fill colour - the white gap in between isn't supposed to be there.

Any help would be much appreciated.