[Help] Convert base

[RaMb]

local floor,insert = math.floor, table.insert
function co(n,b)
    n = floor(n)
    if not b or b == 10 then return tostring(n) end
    local digits = "0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZ"
    local t = {}
    local sign = ""
    if n < 0 then
        sign = "-"
    n = -n
    end
    repeat
        local d = (n % b) + 1
        n = floor(n / b)
        insert(t, 1, digits:sub(d,d))
    until n == 0
    return sign .. table.concat(t,"")
end 

That function use to convert a decimal number to other base (36)
So, this script:

--21093974655
co('21093974655',36) -- return: 9OUT30F
tonumber('9OUT30F', 36) -- return: 4294967295 

Whats wrong here @@
Someone can explain this, thank ._.

From lua manual:

tonumber (e [, base])
Tries to convert its argument to a number. If the argument is already a number or a string convertible to a number, then tonumber returns this number; otherwise, it returns nil.
An optional argument specifies the base to interpret the numeral. The base may be any integer between 2 and 36, inclusive. In bases above 10, the letter 'A' (in either upper or lower case) represents 10, 'B' represents 11, and so forth, with 'Z' representing 35.

[RaMb]

for i = 2,36 do
    local s = co('21093974655',i)
    local n = tonumber(s,i)
    print(n == 21093974655)
end 

Magic number :v :v :v