Saving a graph with ggsave after using ggplot_build and ggplot_gtable

Saving a graph with ggsave after using ggplot_build and ggplot_gtable

I am modifying a graph built with ggplot by altering the data produced by
ggplot_build (for a reason similar to R ggplot empty factor level). As far
as I understand the help I found on this topic, I should be able to save
the result by applying ggplot_gtable and arrangeGrob before calling ggsave
on the results (Saving grid.arrange() plot to file).
However I obtain an error "plot should be a ggplot2 plot", also with this
simple reproductible example:
require('ggplot2')
require('gridExtra')
df <- data.frame(f1=factor(rbinom(100, 1, 0.45), label=c("m","w")),
f2=factor(rbinom(100, 1, 0.45), label=c("young","old")),
boxthis=rnorm(100))
g <- ggplot(aes(y = boxthis, x = f2, fill = f1), data = df) + geom_boxplot()
dd <- ggplot_build(g)
# Printing the graph works:
print(arrangeGrob(ggplot_gtable(dd)))
# Saving the graph doesn't:
ggsave('test.png',arrangeGrob(ggplot_gtable(dd)))
Can anybody explain why this does not work ? Is there a way to use ggsave
after modifying the data by using ggplot_build() ?
(My version of the packages are gridExtra_0.9.1 and ggplot2_0.9.3.1)