What happens if you press PERF before selecting a DRWY?

Hi guys,

Just wondering if anybody knows how the MCDU will respond if I press PERF before filling out a F-PLN and selecting a DRWY?

In all the example screens that I've seen the runway number is always shown, so the manual writers have clearly expected that the pilots will follow SOP, and of course in real life they should do that without fail, but I want to be able to cover every possibility of what somebody might do.

Would the software (in the real aircraft, not some crummy sim version that doesn't work exactly like the real thing) faithfully display the PERF page but with a blank runway field (what I would expect) or would it show some kind or error message?

Thanks for reading and I hope somebody can provide an answer.