Export audio - bug or user error?

Hello, I'm driving myself crazy trying to get audio to export properly via the "export tracks" dialogue. I'm pretty new to Qtractor so might have misunderstood something about the routing - I would be very grateful for any advice.

Basically exporting the master bus (or anything else I've tried) results in an empty audio file. I can workaround this problem by recording the master bus into a new track and then locating the resulting file on the hard drive, but that seems like it should be an unnecessary step.

I've attached a screenshot to show how I have things connected. You can see a newly created exported track in pink, with a thin line where I would expect audio date to be. During the export process the track faders light up and move as you would expect, but the master fader doesn't. When the export completes there is a loud pop and the active faders go into the red.

Any ideas what's going wrong here? I've searched and found nothing of any relevance.

Version 0.9.19 with jack2 by the way.