Infinite loop

[Truman]

# [ ] use a "forever" while loop to get user input of integers to add to sum,
# until a non-digit is entered, then break the loop and print sum
sum = 0

the best I was able to think up doesn't work

while True:
    sum = ''
	sum1 = input("add number: ")
	if sum1.isdigit():
	    sum = sum + sum1
	else:
	    break
		print(sum)

Suggestions are welcome.