Warning when indexing data.frame with FALSE

Sorry, a better example is:

data.frame(a=1)[FALSE]

NULL data frame with 1 rows

data.frame(a=1)[NULL]

NULL data frame with 1 rows

vs

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

Hadley