Warning when indexing data.frame with FALSE

data.frame()[]

NULL data frame with 0 rows

data.frame()[FALSE]

Warning in is.na(nm) : is.na() applied to non-(list or vector)
NULL data frame with 0 rows

data.frame()[NULL]

Warning in is.na(nm) : is.na() applied to non-(list or vector)
NULL data frame with 0 rows

Is this a bug?  I wouldn't have expected the warning in the last two cases.

Regards,

Hadley