polyline with stroke line join type round bug

[nhk15]

Hi
I have found a bug in drawing polyline.

If the stroke width is greater than 2 and stroke line join is round, then it draws an extra line segment from the last point to the second point.
Here are the command line to test

working:
convert -size 800x600 xc:white -fill white -draw "stroke black stroke-width 2 stroke-linejoin round polyline 346,239 403,151 518,141 510,213" line.png
bug:
convert -size 800x600 xc:white -fill white -draw "stroke black stroke-width 3 stroke-linejoin round polyline 346,239 403,151 518,141 510,213" line.png

This happens in Imagick as well.
Hope we can resolve this issue.

Cheers,
Nam