React Native - ios - circle with border - circle background color spilling out of circle

I'm trying to create a circle with a white background in react native and i'm having an issue where the background color of the circle is seen on the outline of the border.

Check out this playground app:https://rnplay.org/apps/TsQ-CA

If you look closely you can see that around the circle there's a thin black line. It's like the border isn't covering the entire background.

Here's the circle style:circle: { borderRadius: 40, width: 80, height: 80, borderWidth: 5, borderColor: 'white', backgroundColor: 'black' }

P.S. this only happens on iOS

Any ideas??Thanks!