data.table : `get` of a non existing column to existing column fails silently

    > d <- data.table(x = 1:5)
    > d[x == 6, y := get("i_9")]
    Error in get("i_9") : object 'i_9' not found

    > d[, y := 1]   # add a new column y
    > d[x == 6, y := get("i_9")]
    <No Error>

get of non existing column i_9 to:

a. non existent column y fails
b. existent column y fails silently

How do we justify this behavior.