How does this png filter sub work?

Question

PNG

image file png 3x3 with all different colors
encoded in idat block 3 scan lines 1st line has filter 1 (sub)
colors encoded in png through photoshop rgb (0,162,232) (185,122,87) (34,177,76)
here is a description of the filter
code

def Raw(i,d):
    if i < 0:
        return 0
    else:
        return d[i]
def unsub(d):    
    dd = b""
    for i,b in enumerate(d):
        bpp = 4
        dd += bytes([b + Raw(i - bpp,d)])
    return dd

where d is a byte string scan line without specifying a filter
d is equal to 00a2e8ffb9d86f006937f500 (in hex) is
12
3 pixels long with 4 channels p g b and
on byte 2 2 pixels this error occurs:

dd += bytes([b + Raw(i - bpp,d)])
ValueError: bytes must be in range(0, 256)

that is, there the number turned out to be more, but as if I did everything according to the specs, but how is it