[R] sum() returns 0 not NA

[will.eagle at gmx.net]

Dear all,

just a stupid R question, since the results puzzle me a bit:

> sum(c(NA,NA), na.rm=TRUE)
[1] 0
>  NA + NA   
[1] NA
> NA + 1
[1] NA
> 

Why does sum(c(NA,NA), na.rm=TRUE) return 0 and not NA?

Thanks in advance,

Will