The camera needs to be set to function without a FX-Mount lens attached. You can do this by changing the following camera setting:

X-A1, X-A2, X-A3, X-A5, X-A10, X-A20, X-E1, X-E2, X-E2s, X-M1, X-Pro1, X-T1, X-T10, X-T100:

Menu -> Shooting Menu -> Shoot Without Lens -> On

X-E3, X-H1, X-Pro2, X-T2, X-T20, X-T3:

Menu -> Setup Menu -> Button/Dial Settings -> Shoot Without Lens -> On