未为“逻辑”和“句号”类定义算术运算符

Question

DT1 <- data.table(Date = NA, Diff = 5)

DT1[!is.na(Date), val := Date + lubridate::days(Diff)]

Error in Date + lubridate::days(Diff) : 
  Arithmetic operators undefined for 'logical' and 'Period' classes:
  convert one to numeric or a matching time-span class.

如果列日期为NA，则val为NA，我应该怎么做？目前，lubridate出错了。