I'm trying to write a very simple program, I want to print out the sum of all the multiples of 3 and 5 below 100, but, an error keeps accuring, saying "invalid literal for int() with base 10:" my program is as follows:

sum = ""
sum_int = int(sum)
for i in range(1, 101):
    if i % 5 == 0:
        sum += i 
    elif i % 3 == 0:
        sum += i
    else:
        sum += ""

print sum

Any help would be much appreciated.