ImageButton not displaying image in WPF

I have the following ImageButton in a ContentView.

<ImageButton Source="Open.png" />

In a UWP project, I have the image in the root of the project with a Build Action of Content. The image shows fine.
In a WPF project, I have the image in the root of the project also with a Build Action of Content. The image does not show.

If I change whether or not it is copied to the output directory makes no difference.

Is there some other folder or build action or other setting that will get this to work for WPF?