@SilentNick
Chances are it says tile
and you've probably tried to access an image tile that hasn't been created. Tiles are created when you use the command PREPARE MATRIX TEXTURE .
If, for instance, you used
PREPARE MATRIX TEXTURE matnum,imgnum,2,2
that means you've divided the image indicated by imgnum into a 2x2 grid where the tiles are number 1 to 4 where tile 1 (or any number less than 1) is in the upper left corner and tile 4 is in the lower right corner.
If you try and tile your matrix with a number greater than 4 in this case, it will cause the error you printed above.
If you never PREPARE MATRIX TEXTURE , you will also get this error.
Enjoy your day.