Unexpected Black Border in Rounded Corner Image with Transparency true

Thumbor request URL

http://localhost:8888/unsafe/500x/filters:round_corner(150,0,0,0,1)/https://raw.githubusercontent.com/thumbor/thumbor/master/example.jpg

Expected behaviour

The image should be rounded with transparent corners and no visible borders.

Actual behaviour

The image is generated with rounded corners and transparency, but a black border appears around the rounded edges, which should not be present.