zonal()'s as.raster = TRUE does not work on some cases

[Nowosad]

See the below reprex:

# remotes::install_github("nowosad/spData") #dev version of spData is required here
library(terra)
#> terra version 1.4.5
elev = rast(system.file("raster/elev.tif", package = "spData"))
grain = rast(system.file("raster/grain.tif", package = "spData"))

# works
z = zonal(elev, grain, fun = "mean", as.raster = FALSE)
z
#>   grain     elev
#> 1  clay 14.80000
#> 2  silt 21.15385
#> 3  sand 18.69231

# fails
z2 = zonal(elev, grain, fun = "mean", as.raster = TRUE)
#> Error in x@ptr$replaceValues(from, to, -1, opt): Not compatible with requested type: [type=character; target=double].
z2
#> Error in eval(expr, envir, enclos): object 'z2' not found

^{Created on 2021-10-02 by the reprex package (v2.0.1)}