Can't receive WM_COPYDATA

I can’t receive WM_COPYDATA in my SDL 2.0.4 app.

I tried using both SDL_AddEventWatch() and the new SDL_SetWindowsMessageHook(OnWMessage, NULL). Both are receiving events; however, neither is receiving the WM_COPYDATA messages that I am sending. I used Window Detective (a spy++ like program) to verify that the target window is in fact receiving WM_COPYDATA from me. It reports that the window is returning 0 from the event.

Any ideas?