Non-Owners insurance- Worthless?

I recently purchased Non-Owners (renter/borrower) Insurance so that I
could rent a ship at Minden, and take friends up in a 2 seater at
Williams. The company was Avemco, and they claimed when I called them,
that Experimental aircraft were covered as long as there was a valid
airworthiness certificate.

I subsequently rented a glider after presenting proof of insurance with
Avemco, and was told that they had contacted Avemco and Experimental
aircraft were not covered. My limited understanding is that many
gliders in the rental pool fall into the experimental category, perhaps
up to half. I guess you can't tell by type of glider either, as it
depends on how they were imported, equipment, etc. (my knowledge is
obviously sketchy here..)

When I complained, to their credit, Avemco allowed me to cancel with a
full refund, but I am glad I caught this. It doesn't seem to be
explicitly stated in the policy or the website.

Does anyone know where a guy can get non-owner's insurance that covers
experimental gliders from a reputable company?

Thanks!