Strange black edge added to exported images. How do I fix that?

Lately I’ve been getting black edges on exported images. I have images – typically a screen shot – that I add rectangles around to highlight something. Then I export the result. But often the exported image has a black edge.
Which is obviously annoying, since I then have to re-edit the image to remove the black line.

Here’s an example:

I took this image (whose URL I have to obfuscate because this forum limits “new users” to two links). and placed it in a new document

Then I sized it larger, and put a red rectangle somewhere. And exported it.

Here’s what it looks like OmniGraffle:

And here’s the result. The exported file has a black edge on the right.

What’s going on? Thanks for the help! :-)