Dplyr: “Error In N(): Function Should Not Be Called Directly”

I am attempting to reproduce one of the examples in the dplyr package but this error message. I am expecting to see a new column n produced with the frequency of each combination. Can someone tell me what I am missing? I triple checked that the package is loaded. Thanks for the help, as always.

 library(dplyr)
# summarise peels off a single layer of grouping
by_vs_am <- group_by(mtcars, vs, am)
by_vs <- summarise(by_vs_am, n = n())
Error in n() : This function should not be called directly