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?