Argument as.integer(NA) to a function C

Hi the list,
I am writing a R function that call a C function. The C function needs 
integers but I do not manage to give a NA integer as argument :

--- C code ---
void essai(int *t){
  Rprintf("\nT0=%i T1=%i T2=%i T3=%i",t[0],t[1],t[2],t[3]);
}

--- R ---
boub <- c(1,2,3,4)
.C("pour",as.integer(boub),NAOK=TRUE)

# T0=1 T1=2 T2=3 T3=4[[1]]
# [1] 1 2 3 4

boub <- c(1,2,NA,4)
.C("essai",as.integer(boub),NAOK=TRUE)

# T0=1 T1=2 T2=-2147483648 T3=4[[1]]
# [1]  1  2 NA  4
--- ---

In the second example, T2=-2147483648 and not NA.

I check the "writing R extension", there is a part that explain that the test 
of NA is not the same between double and integer (NA_INTEGER, ISNA), but I 
did not find explanation on passing NA argument as integer.

Any idea of what is wrong in my code?