[Microbit-Python] error code?... :(020

[Alan]

Hi,

I wrote a small program to draw on the LED matrix, but after setting a few LEDs on I get what looks like an error message on the LEDs:

":(020"

(Frowny-face zero two zero)

It just keeps repeating that and my program stops but there's no stack trace on the python repl. 

Is that a built in hardware error code or something? Has anyone else seen that?

Here's my program:

====8<====

from microbit import *

index = 0

while True:
    if button_a.is_pressed():
        x = index % 5
        y = int(index / 5)
        display.image.set_pixel_value(x,y, not(display.image.get_pixel_value(x,y)))
    
    if button_b.is_pressed():
        index = (index + 1) % 25
 		 	   		  
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