ggmosaic: "Discrete value supplied to continuous scale" error when ggplot2/ggmosaic is imported

Question

In my package, I like to create a mosaic plot. In the interactive session, this works well. However, if I call the function from an fresh R session with devtools::load_all, the error "Discrete value supplied to continuous scale" is raised.

Here is a minimum working example:

test_ggmosaic <- function(dat) {
    dat <- datasets::mtcars
    p <- ggplot2::ggplot(data=dat)+
        ggmosaic::geom_mosaic(
            ggplot2::aes(weight=mpg,x=ggmosaic::product(gear, cyl), fill=mpg)
        ) +
        ggplot2::theme_bw()+
        ggplot2::scale_fill_discrete(guide=ggplot2::guide_legend(reverse=TRUE)
    )
    return(p)
}

Now when this function is defined inside a package, and ggplot2 and ggmosaic are added to the Imports field in the DESCRIPTION file, and I call this function after loading devtools and (via load_all) the package the function is in, I get the error

Fehler: Discrete value supplied to continuous scale

When, on the other hand, I first execute

library(ggplot2)
library(ggmosaic)

and then call the function above, the plot appears (and does not make much sense in this example).