Confusing error message when trying to group by a list column

```r
iis <- data.table(iris)
iis[, letts := .(list(sample(letters[1:4], size = 2))), 1:nrow(iris)]
iis[, .N, .(Species, letts)]

Error in `[.data.table`(iis, , .N, .(Species, letts)) : 
  column or expression 2 of 'by' or 'keyby' is type list. Do not quote column names. Usage: DT[,sum(colC),by=list(colA,month(colB))]
```

"Do not quote column names" seems out of place. 