Rcpp error when using prophet::prophet() instead of library(prophet)

When I don't use `library(prophet)` and instead use the fully-qualified `prophet::prophet`, I get the following in my console:

> ds <- seq(as.Date('2005-04-01'), as.Date('2005-09-03'), by='day')
x <- (1:length(ds))/length(ds)
y <- rnorm(length(ds), x * (1 - x) + 0.5, 0.1)
prophet::prophet(data.frame(ds = ds, y = y), yearly.seasonality = FALSE)

>Error in .Object$initialize(...) :
could not find function "cpp_object_initializer"
failed to create the optimizer; optimization not done
Error in matrix(m$params[[name]], nrow = n.iteration) :
'data' must be of a vector type, was 'NULL'

I'm using the latest revision from master (via `devtools::install_github('facebookincubator/prophet', subdir='R')`)