Python - PyPNG struct.error

import png

data = list()

for r in range(0, 512): # r - row number
    row = list()
    for c in range(0, 512): # c - column number
        if r == c:
            row.append(0) # OK (black)
        else:
            row.append((256*256)-1) # OK (white) (65535)

            #row.append((256*256)) # ERROR - value too big for bitdepth=16
            # struct.error: 'H' format requires 0 <= number <= 65535

            #row.append(-1) # ERROR - value too small for bitdepth=16
            # struct.error: argument out of range

    data.append(row)

out = png.Writer(width=512, height=512, greyscale=True, bitdepth=16)
out.write(open('output.png', 'wb'), data)