Performance test on hummingbird didn't return any results

Hi there,

I get this message : " The performance test didn't return any results. This probably means you're on a local website (which we can't scan) or something went wrong trying to access WPMU DEV. Try again and if this error continues to appear please open a ticket with our support heroes" when i try to run a performance test.
I'm not sure what i'm doing wrong.
Many thanks
Dorothy