Does anybody know how to aggregate by NA in R.

If you take the example below

```
a <- matrix(1,5,2)
a[1:2,2] <- NA
a[3:5,2] <- 2
aggregate(a[,1], by=list(a[,2]), sum)

```

The output is:

```
Group.1 x
2       3

```

But is there a way to get the output to include NAs in the output like this:

```
Group.1 x
2       3
NA      2

```

Thanks