Understanding variables

In the previous exercise you saw that disp can be mapped onto a color gradient or onto a continuous size scale.

Another argument of aes() is the shape of the points. There are a finite number of shapes which ggplot() can automatically assign to the points. However, if you try this command in the console:

ggplot(mtcars, aes(wt, mpg, shape = disp)) +
  geom_point()

It gives an error. What does this mean?