Can't select helix

I've downloaded a Nyloc nut from 3D Content Central, nicepart with enough configurations.
Now i like to cut some thread into it, select a face, create asketch, convert entity 'hole circle' and insert a helix.
So far no problem.
When i try to create a 'plane normal to curve' I can't select thehelix.

When i create 'my own' nut it's no problem to select the helix encreate the plane.

Somebody know why ?
In both the properties I can't find any restrictions regardinghelix.
I've also tried to do a 'normal extruded cut' which is no problem.

Greetings !

SolidworksParts And Features