Just found this on the RB site...not sure if it's true.
Stocks that trade under prospectus cannot be margined (or shorted). IPOs trade under prospectus for 30 days after initial trade - that means if you buy the shares in the secondary market 25 days AFTER it goes public, your broker needs to send you a prospectus. After 30 days it trades like any other stock - without prospectus.
KREM or any other recent IPO cannot be shorted for 30 days after IPO.