WIA Scanner Minidriver - Preview Not Working

I've written a WIA minidriver and I cannot get the preview button to
work. When I click the preview button (on the standard UI) I get a
message that start, "An error occurred during preview. If the scanner
is being used..." I'm not currently using the scanner and I believe
I've defined WIA_DPS_PREVIEW correctly, but in my logs I don't see the
driver trying to scan at all when I click preview. Does anyone have any
idea why this might be?

Thanks in advance,
Mike