Getting weird black lines on custom brush and Track Direction

[simon2]

Hi,

I'm trying to make a custom brush with a gradient and then use the paintbrush tool with the "Track Direction" dynamics to give the gradient to a line that I'm drawing. The brush works fine without dynamics, but when I apply the "Track Direction" dynamics, I get these weird artefacts. Per the image below:



I attach my brush, in case it's something I've done wrong making the brush.