Surprising behavior of letters[c(NA, NA)]

Consider this:

letters[c(2, 3)]

[1] "b" "c"

letters[c(2, NA)]

[1] "b" NA

letters[c(NA, 3)]

[1] NA  "c"

letters[c(NA, NA)]

[1] NA NA NA NA NA NA NA NA NA NA NA NA NA NA NA NA NA NA NA NA NA NA NA NA NA
[26] NA

The result is a 2-vector in each case until we get to c(NA, NA) and
then it unexpectedly changes from returning a 2-vector to returning a
26-vector.  I think most people would have expected that the answer
would be c(NA, NA).

Statistics & Software Consulting
GKX Group, GKX Associates Inc.
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