How do I capture a screen with xwd?

I need to capture a window with xwd:

$ xwd -out /home/me/dump

I run it, I get a crosshair pointer, click on the target window and the screenshot is dumped. I can view it with xwud.

Now, I need to do that non-interactively.

$ xprop WM_NAME

I get a crosshair pointer, click on the target window and the output tells me:

WM_NAME(STRING) = "Name of application"

So I run:

$ xwd -name "Name of application" -out /home/me/dump

Then I try to view it with xwud and the result is a completely black screen. There seems to be a thin gray frame around it.

What am I doing wrong?