replacement has 1 row, data has 0

> datap <- data.frame(p=1:10)
> datap
    p
1   1
2   2
3   3
4   4
5   5
6   6
7   7
8   8
9   9
10 10
> datap$p0 <- c('NA')
> datap
    p p0
1   1 NA
2   2 NA
3   3 NA
4   4 NA
5   5 NA
6   6 NA
7   7 NA
8   8 NA
9   9 NA
10 10 NA
> datap[is.na(datap$p),]$p0 <- '1'
Error in `$<-.data.frame`(`*tmp*`, p0, value = "1") :
  replacement has 1 row, data has 0