[R] apply question

[clsnyder]

Why does one method work, and the other not?

attach(iris)
head(iris)

mean(Sepal.Length)
[1] 5.843333

apply(iris, Sepal.Length, mean)
"Error in if (d2 == 0L) { : missing value where TRUE/FALSE needed"

I seem to get this error frequently with the apply functions but other
"manual" methods work normally on the same data. 

TIA

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