Convert a number to Excel’s base 26

OK, I'm stuck on something seemingly simple. I am trying to convert a number to base 26 (ie. 3 = C, 27 = AA, ect.). I am guessing my problem has to do with not having a 0 in the model? Not sure. But if you run the code, you will see that numbers 52, 104 and especially numbers around 676 are really weird. Can anyone give me a hint as to what I am not seeing? I will appreciate it. (just in case to avoid wasting your time, @ is ascii char 64, A is ascii char 65)

def toBase26(x):
    x = int(x)
    if x == 0:
        return '0'
    if x < 0:
        negative = True
        x = abs(x)
    else:
        negative = False
    def digit_value (val):
        return str(chr(int(val)+64))
    digits = 1
    base26 = ""
    while 26**digits < x:
        digits += 1
    while digits != 0:
        remainder = x%(26**(digits-1))
        base26 += digit_value((x-remainder)/(26**(digits-1)))
        x = remainder
        digits -= 1
    if negative:
        return '-'+base26
    else:
        return base26

import io    
with io.open('numbers.txt','w') as f:
    for i in range(1000):
        f.write('{} is {}\n'.format(i,toBase26(i)))

So, I found a temporary workaround by making a couple of changes to my function (the 2 if statements in the while loop). My columns are limited to 500 anyways, and the following change to the function seems to do the trick up to x = 676, so I am satisfied. However if any of you find a general solution for any x (may be my code may help), would be pretty cool!

def toBase26(x):
    x = int(x)
    if x == 0:
        return '0'
    if x < 0:
        negative = True
        x = abs(x)
    else:
        negative = False
    def digit_value (val):
        return str(chr(int(val)+64))
    digits = 1
    base26 = ""
    while 26**digits < x:
        digits += 1
    while digits != 0:
        remainder = x%(26**(digits-1))
        if remainder == 0:
            remainder += 26**(digits-1)
        if digits == 1:
            remainder -= 1
        base26 += digit_value((x-remainder)/(26**(digits-1)))
        x = remainder
        digits -= 1
    if negative:
        return '-'+base26
    else:
        return base26

from Convert a number to Excel’s base 26