s4 objects in j groups are wrong #4415

library(data.table)
library(lubridate)
dt <- data.table(period_unit = c("days", "months", "weeks", "years"),
                 period_length = rep(1, times = 4))

dt[ , by = period_unit, do.call(.BY$period_unit, list(period_length))]
#    period_unit          V1
# 1:        days 1d 0H 0M 0S
# 2:      months 1d 0H 0M 0S
# 3:       weeks 1d 0H 0M 0S
# 4:       years 1d 0H 0M 0S