Why am I getting this error? ("Cannot create PyString with non-byte value")

So I just copied and pasted a code, my professor had on his lecture slide, on to my processing (which is in python mode).

planet_x = [] # list of planet centre x- coordinates
planet_y = [] # list of planet centre y- coordinates
planet_diameter = 50 # planet diameter in pixels

def setup ():
# create 500 x500 canvas with black backdrop
    size (500 ,500)
    background (0 ,0 ,0)
    global planet_x
    global planet_y
    # initialize the sun & two other planets
    planet_x = [250 ,100 ,400]
    planet_y = [250 ,400 ,100]
def draw ():
    global planet_x
    global planet_y
    global planet_diameter
    background (0 ,0 ,0)
    # draw the sun
    fill (255 ,255 ,0)
    ellipse ( planet_x [0] , planet_y [0] , planet_diameter , planet_diameter )
    # all other planets
    for i in range (1 , len ( planet_x )):
        fill (0 ,0 ,255)
        ellipse ( planet_x [ i ] , planet_y [ i ] , planet_diameter , planet_diameter )
def mouseClicked():
    global planet_x
    global planet_y
    # add mouse click coords as new planet to draw
    planet_x.append ( mouseX )
    planet_y.append ( mouseY )
def keyPressed ():
    global planet_x
    global planet_y
    # remove last drawn planet (do not remove the sun )
    if key == ’r’:
        planet_x.pop()
        planet_y.pop()

but it's giving me an error saying "Cannot create PyString with non-byte value". The error seems to pop up when I include mouseClicked and keyPressed