Greek letters in ggplot strip text

I'm trying to override the text in some ggplot strips to incorporate Greek characters. Here's some sample data, and the base for the plot.

dfr <- data.frame(
   x = rep(1:10, times = 6),
   y = runif(60),
   fx = rep(c("foo", "bar"), each = 30),
   fy = rep(c("alpha", "beta", "gamma"), each = 10, times = 2)
)

p <- ggplot(dfr, aes(x, y)) + geom_point()

My first attempt at a plot has no Greek in the strip labels.

 p + facet_grid(fy ~ fx)

I gather that I'm supposed to add a labeller argument to facet_grid to override the text. I presumed that this should spit out an expression to handle the greek characters, but my code just throws an error when the graphic is printed.

lbl <- function(variable, value)
{
   if(variable == "fy") parse(text=as.character(value)) else value
}
p + facet_grid(fy ~ fx, labeller = lbl)


Error in aperm(X, c(s.call, s.ans)) : 
  unimplemented type 'expression' in 'aperm'

How should I be creating the strip labels?