FORTH for the SHARP PC-E500 (S)

[Helix]

I'm still learning Forth, following the book Starting Forth 2nd edition. (I know it's outdated, but it's didactic and enjoyable. I have also the Forth Programmer's Handbook for a more modern reference.)

I have a problem with the KEY? word. If no key is pressed, it returns "error -57" instead of 0.
The same problem occurs with EKEY? Furthermore, if a key is pressed, EKEY? returns 255. I think it should be -1.