sum() returns 0 not NA

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