Thanks Troy
I haven't been using a T-adapter. The problem is the William Optics advertises this unit as follows:
‘designed to cover full size DSLR cameras with clear lens aperture of 50 mm'. My Nikon D40 is a full size DSLR.
I have emailed Tim at William Optics. I am hoping they they will give me a store credit. On their website they have a strong customer service committment: “two-year, no fuss warranty and constant customer service!”
Fingers crossed
Cheers.