How can I use a variable in aes() with ..count.. in ggplot2?

Question

I'm trying to do this

library(ggplot2)

foo <- 4
ggplot(mtcars, aes(x = wt, y = ..count.. + foo)) + geom_histogram()

And I'm getting

Error in eval(expr, envir, enclos) : object 'foo' not found

Oddly, this works

ggplot(mtcars, aes(x = wt + foo)) + geom_histogram()

Why is it that I can add foo to wt in aes(), but not to ..count..?