geom_density gives confusing error when a group has only one observation

[wch]

dat <- data.frame(x=rnorm(7), g=c("a","a","a","b","b","b","c"))
ggplot(dat, aes(x=x, colour=g)) + geom_density()
#Error in rowSums(vapply(x, is.finite, logical(nrow(x)))) : 
#  'x' must be an array of at least two dimensions
#Error in UseMethod("scale_dimension") : 
#  no applicable method for 'scale_dimension' applied to an object of class "NULL"


# For an even simpler version:
dat <- data.frame(x=3, g="a")
ggplot(dat, aes(x=x)) + geom_density()

[wch]

It actually gives this error when there are < 3 observations:

# Error
dat <- data.frame(x=3)
ggplot(dat, aes(x=x)) + geom_density()

# Error
dat <- data.frame(x=3:4)
ggplot(dat, aes(x=x)) + geom_density()

# OK
dat <- data.frame(x=3:5)
ggplot(dat, aes(x=x)) + geom_density()