ggplotly() does not render empty plots from qplot() grid

Trying to run a 3x3 qplot() grid through ggplotly()—qplot() renders a fine static plot:

library(plotly)

mtcars$gear <- factor(mtcars$gear,levels=c(3,4,5),
  	labels=c("3gears","4gears","5gears")) 
mtcars$am <- factor(mtcars$am,levels=c(0,1),
  	labels=c("Automatic","Manual")) 
mtcars$cyl <- factor(mtcars$cyl,levels=c(4,6,8),
   labels=c("4cyl","6cyl","8cyl")) 

qplot(hp, mpg, data=mtcars, shape=am, color=am, 
           facets=gear~cyl, size=I(3), 
           xlab="Horsepower", ylab="Miles per Gallon")

But then when run through ggplotly(), the mid-right plot isn't rendered:

ggplotly(qplot(hp, mpg, data=mtcars, shape=am, color=am, 
           facets=gear~cyl, size=I(3), 
           xlab="Horsepower", ylab="Miles per Gallon"))

Is this a bug or expected? Any suggestions on a way to force it to show up?

Thanks.

Reproduce el ejemplo proporcionado de qplot() y ggplotly() con los datos mtcars y sus facetas 3x3 por gear y cylinder. Comienza por la ruta de conversión de ggplotly() y compara la salida estática y la interactiva; se considera terminado cuando cada faceta, incluido el panel vacío, se renderiza de forma coherente o el comportamiento está documentado como se esperaba.