how to draw shapes in Preview with no fill

This used to work. Somehow it is turned off and I can't see how to switch it back.

Using Preview, I want to draw a shape on a .png with 'no fill' - IOW transparent fill.

I'm running OS X 10.11.5.

Are there docs for this that I'm not seeing? I looked...

My steps:

Using OS X 10.11.5, in Finder, double-click a .png. Preview opens.

Click the 'show markup toolbar' icon. Markup toolbar opens.

Click the 'shapes' icon. Shapes menu opens.

Click the square shape icon.

Expected result:

A square shape opens with 'transparent' fill - so I can still see the .png image inside the shape.

Actual result:

A square shape opens with fill that overwrites the content of the .png.

There should be a 'no fill' selection somewhere. Where?