It seems to occur whenever PALETTE value is > 255

Maybe this will help or not, but thought I'd post me debugging experience on VORTEX.BAS


I narrowed the 'illegal error' happening down to line 2, and occured only when the LEN(a$) / 3 egualed 255.6666666666667, so I played with PALETTE in SCREEN 15 and it happens everytime > 255 value is given.

Change line 2 to this and it works without the error.

2
IF LEN(a$) / 3) > 255 THEN
' PRINT LEN(a$ / 3)
ELSE
PALETTE (LEN(a$) / 3), 0
END IF