UKHeliBob:
This is causing the squares. Why the program should print it I do not know.Try this example from the Instructables page
When I run this code, I don't get anything on my Serial Monitor. Since my cards don't correspond to the card of this code, (I thought) I should get the text "Not allowed!", but nothing shows up.
By the way, when I replace the '6' for '-1' I got mirrored question marks instead of squares. This does not solve anything, but maybe it helps someone understanding what goes wrong.