[R] Problem Subsetting Rows that Have NA's

[BooBoo]

This has every appearance of being a bug. If it is not a bug, can 
someone tell me what I am asking for when I ask for "x[x[,2]==0,]". Thanks.

 > #here is the toy dataset
 > x <- rbind(c(1,1),c(2,2),c(3,3),c(4,0),c(5,0),c(6,NA),
+   c(7,NA),c(8,NA),c(9,NA),c(10,NA)
+ )
 > x
       [,1] [,2]
  [1,]    1    1
  [2,]    2    2
  [3,]    3    3
  [4,]    4    0
  [5,]    5    0
  [6,]    6   NA
  [7,]    7   NA
  [8,]    8   NA
  [9,]    9   NA
[10,]   10   NA
 >
 > #it contains rows that have NA's
 > x[is.na(x[,2]),]
      [,1] [,2]
[1,]    6   NA
[2,]    7   NA
[3,]    8   NA
[4,]    9   NA
[5,]   10   NA
 >
 > #seems like an unreasonable answer to a reasonable question
 > x[x[,2]==0,]
      [,1] [,2]
[1,]    4    0
[2,]    5    0
[3,]   NA   NA
[4,]   NA   NA
[5,]   NA   NA
[6,]   NA   NA
[7,]   NA   NA
 >
 > #this is more what I was expecting
 > x[which(x[,2]==0),]
      [,1] [,2]
[1,]    4    0
[2,]    5    0
 >