Diesel piston engined aircraft must be fueled by AVGAS, according to EASA

[Andy_RR]

Not being picky, but according to CS 23.1557 Miscellaneous markings and placards;

(1) Fuel filler openings must be marked or near the filler cover with –
(i) For reciprocating engine powered aeroplanes –
(A) The word „Avgas“; and
(B) The minimum fuel grade.

although I'm sure it's just an oversight...

[LowNSlow]

Taking it one step further:

If you don't have an Avgas sticker next to your tank caps will the CAA Sticker police book you?