vlcsnap - tiff is all black

[BGDavis]

I'm using VLC snap to capture frames from video, both 4K and 1080p/60.
With capture set to JPEG and PNG, there are no problems.
But with capture set to TIFF, although the preview thumbnail shows up when I do the capture, the resulting TIFF file is unreadable. Cant' read it with PhotoShop, FastStone, MS Viewer, etc.
A Tiff file seems to have been created; it has a .tif file name extension, it's about 12MB (similar in size to PNG), but can't be read.
Adobe says it's "wrong kind of file." On FastStone it's a black screen.
Any ideas?