data.table merge - by的列名是必需的

Question

我正在合并两个data.tables（版本1.9.4）

library(data.table)

dt1 <- data.table(
  key1 = rep(c("A", "B"), each = 2),
  key2 = c("apple", "orange", "kiwi", "mango"),
  vals = 1:4
)
setkey(dt1, key1, key2)

dt2 <- data.table(
  key_one = rep(rev(c("A", "B")), each = 3),
  key_two = rev(c("apple", "orange", "kiwi", "mango", "fig", "lemon")),
  more_vals = 20:15
)
setkey(dt2, key_one, key_two)

这可以按预期工作

dt1[dt2]

这会返回错误

merge(dt1, dt2)
Error in merge.data.table(dt1, dt2) : 
A non-empty vector of column names for `by` is required.

我是否错误地使用合并？