error with as.factor raster

Dear List members,

I am trying to map the habitat suitability of Nephrops and one of my predictor is a categorical variable.

However when I utilised the command "as.factor" (before to create my rasters stack) I get the error message "Error in 1:ncol(r) : argument of length 0".

Could anyone help me?

r4 <- raster("C:/POSTDOC/NEPHROPS_Habitat_Mapping/NEPHROPS_HabMod_PAPER1/Scotland/IN_eunis_sed.tif")
plot(r4)
is.factor(r4)

[1] TRUE

as.factor(r4)

class       : RasterLayer
dimensions  : 250, 413, 103250  (nrow, ncol, ncell)
resolution  : 0.01484784, 0.01484784  (x, y)
extent      : -7.774709, -1.642552, 54.95371, 58.66567  (xmin, xmax, ymin, ymax)
coord. ref. : +proj=longlat +datum=WGS84 +no_defs +ellps=WGS84 +towgs84=0,0,0
data source : C:\POSTDOC\NEPHROPS_Habitat_Mapping\NEPHROPS_HabMod_PAPER1\Scotland\IN_eunis_sed.tif
names       : IN_eunis_sed
values      : 1, 6  (min, max)
attributes  :
Error in 1:ncol(r) : argument of length 0

levels(r4)

[[1]]
   ID    OBJECTID Value Count sediment
   0           1               1         2716        R
    1           2               2         2249       CS
    2           3               3         2647       MS
    3           4              4         6819        M
    4            5             5          889      MXS
    5            6             6         3647        S


Thank you very much in advance.
Best Regards,
Valentina



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