Why does Linq SUM of nullables return nullable

[matthiaslischka]

The return type of the Linq Sum() method on a list of nullables is nullable, even though the sum will never be null:

int? sum = new[] { (int?)null , (int?)null }.Sum(); // = 0
int? sum2 = new[] { 1, 2, (int?)null }.Sum(); // = 3

So why even have the return type nullable then? I find that confusing as I would have expected the sum to be null if any element is null, just like with regular addition:

int? sum3 = 1 + 2 + (int?)null; // = null