Subset dataframe after addNA of an factor

I’ve got a dataframe with a continuous variable x and a grouping factor. I need to add NA as a factor level for some reason. As a result, the data is:

df <- data.frame(x= 1:4, group= factor(c(NA, 1, 1, 2)))
df$group <- addNA(df$group)

How can I now subset the data in the "group" variable for the NA values? I attempted:

df[df$group == "NA", ]
df[df$group == "<NA>", ]
df[is.na(df$group), ]
df[df$group == levels(df$group)[3], ]

My expected output contains all rows where df$group has factor level NA, i.e. data.frame(x= 1:4, group= factor(c(NA, 1, 1, 2)))[1, ].

I do need to add NA as factor level since it is pretty handy in my situation (see here for one case where it is useful).

>Solution :

You can try this approach:

df[is.na(as.character(df$group)),]
  x group
  1  <NA>

moving the variable to character before looking for na values