#4353: tweak to pass R 3.1 where is.na(NULL) issued a warning
#4353: tweak to pass R 3.1 where is.na(NULL) issued a warning

Rdatatable/data.table,vlulla/data.table,jangorecki/data.table,Rdatatable/data.table,jangorecki/data.table,Rdatatable/data.table,Rdatatable/data.table,vlulla/data.table,jangorecki/data.table,vlulla/data.table,vlulla/data.table,jangorecki/data.table