Handling NA values

Hello again,

I have a question on who sum() handle the NA values.

sum(c(NA, 1), na.rm = TRUE)

[1] 1


I understand this. However could not agree with following:

sum(c(NA, NA), na.rm = TRUE)

[1] 0


Where this '0' is coming from? Should not it be NA itself?

Thanks and regards,