Shaded Hidden Line generating unwanted triangles

[avro-designs]

I've used the this technique a few times and now I'm getting strange triangles rendered all over wall objects.

Foreground render: Hidden Line

Background render: Artistic Renderworks; Lines and Shadow

 

 

I double checked smoothing angle and other settings, but nothing gets rid of these triangles:

• When I switch to only HL the problem goes away.
• When I switch to Renderworks Final for background the problem also goes away.

 

So it seems this is caused by the Artistic Renderworks option.