ggplot2: stat_summary throws error when trying to pass function argument as parameter, rather than hard-coding

I'm getting an error when I try to pass a parameter into the round function within stat_summary (even though the analogous code works with, say, geom_text). Here's an example:

# Fake data
set.seed(5)
dat = data.frame(group=rep(c("A","B"),each=10), val=rnorm(20))

We'll try to set the number of decimal places for value labels using a parameter, rather than hard-coding it:

places = 2

ggplot(dat, aes(group, val)) +
  stat_summary(fun.y=mean, geom="text", aes(label=round(..y.., places)))

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

The following two examples work fine, however.

ggplot(dat, aes(group, val)) +
  stat_summary(fun.y=mean, geom="text", aes(label=round(..y.., 2)))

ggplot(dat, aes(group, val)) +
  geom_text(aes(label=round(val, places)))

I ran into this problem while trying to write a ggplot function. At first I thought the problem involved ggplot not getting the parameter from the function environment, but the example above suggests that's not the issue. For completeness, below is a simplified example of the function, along with the error message. The function works fine if I hard-code the digits argument to round, rather than trying to pass the places parameter.

pp1 = function(data, group, var, places=2, e=1.5) {

  ggplot(data, aes_string(group, var)) +
    geom_boxplot() +
    stat_summary(fun.y=mean, geom="text", aes(label=round(..y.., places))) +
    scale_y_continuous(limits = e * range(data[,var]))

}

pp1(dat, "group","val")

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

I'm hoping to find out whether I'm doing something wrong and how I can get the desired behavior.

I'm running R 3.2.3 and ggplot2 2.1.0 on OS X 10.10.5.