Error: scanning slides with CanoScan 9950F

I just received my 9950F scanner. Works fine scanning prints. I tried
placing a couple of slides in the appropriate tray and get the
following error:

Cannot scan for these reasons:
- The original may not be set correctly
- The protective sheet may be in place
- The lamp may not be warmed up (needs a few minutes for adjusting lamp
on low temp).

Scanner driver will be closed
Code: 2,178,0

I have made sure that the originals are set correctly etc.etc. Lamp has
been ON for the last 30 minutes.

Any clues?
Thanks.