ggsave(): Error in UseMethod("grid.draw") : no applicable method for 'grid.draw' applied to an object of class "character"

I am trying to save a plot with ggsave(). I enter the following:

library(ggplot2)

Test = data.frame("X" = seq(1, 10, 1), "Y" = 2*seq(1, 10, 1))

P = ggplot(
    Test, aes(x=X, y=Y))+
    geom_line()

ggsave(P, "test.pdf", device = "pdf")

But get the error:

Saving 7 x 7 in image
Error in UseMethod("grid.draw") : 
  no applicable method for 'grid.draw' applied to an object of class "character"