I haven't had a chance to track this one down in the source code yet.
I am reporting it here in case someone else has time to look at it.
When using a logical vector as an index on a vector, things work as
expected until you get to a list for which the indices are all
FALSE.> tn <- 1:2
> tn[c(T,T)]
[1] 1 2> tn[c(F,T)]
[1] 2> tn[c(F,F)]
integer(0)> tt <- list(foo = 1, bar = 2)
> tt[c(T,T)]
$foo
[1] 1
$bar
[1] 2
> tt[c(F,T)]
$bar
[1] 2
> tt[c(F,F)]
Error: attempt to set an attribute on NULL
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